Circuit board testing apparatus and method for testing a circuit board

ABSTRACT

A circuit board testing apparatus for testing continuity and/or short-circuit of wirings formed on a circuit board, includes an electromagnetic wave irradiator which irradiates first terminals of the wirings with an electromagnetic wave so that electrons are discharged from the first terminals by photoelectric effect. Discharged electrons are trapped by an electrode which is electrically biased to have a higher potential than that of the second terminals of the wirings, thereby causing an electric current to flow through the wirings via the electrode. Existence of open-circuit and/or short-circuit of the wirings is judged based on the current flowing the wirings.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a circuit board testing apparatus andmethod for testing such electric state as continuity, open-circuit,short-circuit, of a number of wirings formed on a circuit board.

[0002] It should be appreciated that this invention is applicable totesting of electric state of wirings formed on any of a variety ofcircuit boards or substrates such as printed circuit boards, flexiblecircuit boards, multi-layer circuit boards, glass substrates for use inliquid crystal display or plasma display panels, and film carriers foruse in semiconductor packages, and that the term “circuit boards” asused in this specification may be any of these variety of boards.

[0003] Circuit boards are formed with a wiring pattern by a number ofwirings thereon. There have been proposed a number of testing apparatusto test whether the wiring pattern has been formed as designed. Recenttrend of making small-sized and light-weighted electronic devicesnecessitates arranging a wiring pattern in a complex manner in a smallarea. Thus, it is difficult to test open-circuits and short-circuits ofwirings by direct contact of probes with the circuit board. Therefore,there has been proposed a contactless testing apparatus to test electricstate of a wiring pattern such as open circuit without causing directcontact of probes with a minute conductive pad.

[0004] For example, Japanese Patent No. 3080158 discloses this type ofapparatus which is adapted for testing an open or short-circuit of awiring formed on a circuit board. In the apparatus, specifically, anelectromagnetic wave is irradiated onto a pad connected to each wiringof a circuit pattern formed on a circuit board to thereby causedischarge of electrons from the pad owing to photoelectric effect. Theopen or short-circuit of the wiring is tested based on an electriccurrent which is caused by discharged electrons to run into a ground(GND) plane or external metallic plate capacitively coupled.

[0005] Japanese Unexamined Patent Publication No. 8-278342 discloses aprinted board testing apparatus which detects electrons discharged in aspace owing to photoelectric effect to test continuity or open-circuitof wirings of the printed board. Specifically, an electric charge sensorand an electromagnetic wave generator are movably provided above theprinted board with a specified gap or distance therebetween. Theelectric charge sensor and the electromagnetic wave generator arerelatively moved over the printed board to scan discharged electrons.The electric state of wirings are judged based on detected changes ofthe electric current.

[0006] The aforementioned conventional arts are suffered from thefollowing drawbacks. In the conventional art, an electromagnetic wave ismerely irradiated onto a pad or wirings. Electrons which are dischargedowing to photoelectric effect upon irradiation are returned to the padand wirings, or dispersed in the space without being utilized for thetesting.

[0007] Further, discharged electrons form a spatial charge region, andlower the electron discharging efficiency of the photoelectric effect.Even if electrons are discharged instantaneously owing to photoelectriceffect, accordingly, current flowing in the ground plate or externalmetallic plate cannot be measured with reliability. Thus, it isdifficult to accomplish stable and precise test efficiency.

[0008] In the apparatus of Japanese Unexamined Patent Publication No.8-278342, further, the electric charge sensor and the electromagneticwave generator are moved relatively to the printed board to scandischarged electrons, which consequently increases the size of theapparatus. It will be seen that in the case of producing a vacuumedspace between the printed board and the electric charge sensor and theelectromagnetic wave generator, a larger-sized vacuuming unit isrequired.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to provide a circuitboard testing apparatus and testing method which are free from theproblems residing in the prior art.

[0010] It is another object of the present invention to provide acircuit board testing apparatus and testing method which can judge thecontinuity and/or short-circuit of wirings formed on a circuit boardaccurately and stably.

[0011] It is still another object of the present invention to provide acircuit board testing apparatus and testing method which can assure moreefficient testing of the continuity and/or short-circuit of wiringsformed on a circuit board.

[0012] It is yet another object of the present invention to provide acircuit board testing apparatus which is small in size and enables testof wirings in a short time.

[0013] It is still further object of the invention to provide a circuitboard testing apparatus and testing method that enables testing ofwirings formed on the circuit board with the test signals being derivedfrom the circuit board without mechanical mechanic contact at least onone side of the circuit board.

[0014] According to an aspect of the present invention, a circuit boardtesting apparatus is adapted for testing continuity and/or short-circuitof wirings formed on a circuit board. First terminals of the wirings areirradiated with an electromagnetic wave so that electrons are dischargedfrom the first terminals by photoelectric effect. Discharged electronsare trapped by an electrode which is electrically biased to have ahigher potential than that of the second terminals of the wirings,thereby causing an electric current to flow through the wirings via theelectrode. Existence of open-circuit and/or short-circuit of the wiringsis judged based on the current flowing the wirings. According to anembodiment of the invention, the first terminals are irradiated with theelectromagnetic wave alternatively, one at a time. Also, the secondterminals of the wirings are supplied with voltage one at a time.Alternatively, a voltage may be supplied to the second terminal of awiring adjacent to a selected wiring of which first terminal isirradiated by the electromagnetic wave.

[0015] For the testing of a circuit board having wirings including apair of terminals formed on a surface of the circuit board and anelectric conductor formed on the surface of the circuit board or insidethe circuit board and connected to the pair of terminals, there may bepreferably provided a second electrode to be capacitively coupled to theelectric conductor. The continuity of the electric conductors is judgedbased on a current value when the first terminal of a target wiring isirradiated and another current value when the second terminal of thetarget wiring is irradiated.

[0016] For the testing of circuit board including wirings havingelectric conductors formed on the surface or inside of the circuit boardand electrically connected to respective first and second terminals,there may be preferably provided a second electrode to be capacitivelycoupled to the electric conductors. The short-circuit between wirings isjudged based on a current value when the one of the first terminals isirradiated and another current value when another first terminal isirradiated.

[0017] Alternatively, the electromagnetic wave may be collectivelyirradiated onto the first terminals of the wirings. In this case, it maybe preferable to provide a power source having a first pole connected tothe electrode and a second pole connected to the second terminal of awiring selected for the test. The second terminals of the wirings otherthan the target wiring may be connected to the first pole of the powersource.

[0018] Alternatively, the second terminal of a target wiring may beconnected to the second pole of the power source by way of a currentdetector for detecting a current of the target wiring while the secondterminals of the wirings other than the target wiring are connected tothe second pole of the power source bypassing the current detector.

[0019] It may be preferable to enclose the first terminals of thewirings within an airtight closed space, and depressurizes the closedspace. The degree of depressurization is preferably 10-2 atm.

[0020] A circuit board testing apparatus or method according to thepresent invention provide accuracy and efficiency in the testing of thecontinuity and/or short-circuit of wirings on a circuit board becauseelectrons discharged by photoelectric effect are captured by theelectrically biased electrode and cause an enhanced electric currentthrough the wirings connected with the electrode.

[0021] These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments/examples with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagram showing a circuit board testing apparatus inaccordance with a first embodiment of the present invention;

[0023]FIG. 2 is a block diagram showing an electric arrangement of thetesting apparatus shown in FIG. 1;

[0024]FIG. 3 is a flowchart showing operations of the testing apparatusshown in FIG. 1;

[0025]FIG. 4 is a flowchart showing operations of an open circuittesting by the testing apparatus shown in FIG. 1;

[0026]FIG. 5 is a timing chart in the open circuit test;

[0027]FIG. 6 is a flowchart showing operations for a short circuittesting by the testing apparatus shown in FIG. 1;

[0028]FIG. 7 is a diagram showing a circuit board testing apparatus as amodification of the first embodiment;

[0029]FIG. 8 is a diagram showing a circuit board testing apparatus inaccordance with a second embodiment of the invention;

[0030]FIG. 9 is a block diagram showing an electric arrangement of thetesting apparatus shown in FIG. 8;

[0031]FIG. 10 are graphs respectively showing changes of a potential ata wiring, a current detected by a current detecting section, and anamount of electric charges charged at a capacitor upon irradiation of anelectromagnetic wave shown in FIG. 8;

[0032]FIG. 11 is a flowchart showing operations of the testing apparatusshown in FIG. 8;

[0033]FIG. 12 is a flowchart showing operations for a wiring test by theapparatus in accordance shown in FIG. 8;

[0034]FIG. 13 is a flowchart showing a wiring test operation by theapparatus shown in FIG. 8, altered from the operation shown in FIG. 12;

[0035]FIGS. 14A and 14B are sets of graphs each set showing changes of apotential at a wiring, a current detected by a current detectingsection, and an amount of electric charges as the integration of thecurrents detected by the current detecting section while theelectromagnetic wave is being irradiated with the irradiation isswitched from one terminal to another;

[0036]FIG. 15 is a diagram showing a circuit board testing apparatus asa first modification of the second embodiment;

[0037]FIG. 16 is a diagram showing a circuit board testing apparatus asa second modification of the second embodiment;

[0038]FIG. 17 is a diagram showing a circuit board testing apparatus inaccordance with a third embodiment of the invention;

[0039]FIG. 18 is a block diagram showing an electric arrangement of thetesting apparatus shown in FIG. 17;

[0040]FIG. 19 is a flowchart showing operations of the testing apparatusshown in FIG. 17;

[0041]FIG. 20 is a flowchart showing operations of an open circuittesting by the testing apparatus shown in FIG. 17;

[0042]FIG. 21 is a diagram showing a testing apparatus as a firstmodification of the third embodiment;

[0043]FIG. 22 is a diagram showing a testing apparatus as a secondmodification of the third embodiment;

[0044]FIG. 23 is a flowchart showing operations of an open/short circuittesting by the apparatus shown in FIG. 22;

[0045]FIG. 24 is a diagram showing a testing apparatus as a thirdmodification of the third embodiment;

[0046]FIG. 25 is a flowchart showing operations of an open/short circuittest by the apparatus shown in FIG. 24; and

[0047]FIG. 26 is a diagram showing a testing apparatus as a fourthmodification of the third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] Referring to FIGS. 1 and 2 showing a circuit board testingapparatus in accordance with a first embodiment of the invention, acircuit board testing apparatus is adapted to test a circuit board 10 onwhich a semiconductor chip is to be mounted according to C4 (ControlledCollapse Chip Connection) package method.

[0049] As shown in FIG. 1, the circuit board 10 is formed with a numberof wirings 12, 121, 122 on a base plate 11. Each wiring 12, 121 or 122includes a pad portion 12 a, 121 a or 122 a which is formed on one orupper surface of the base plate 11 in correspondence to a pad portion towhich a semiconductor chip is connected, a ball grid portion 12 b, 121 bor 122 b which is formed on the opposite or bottom surface of the baseplate 11, and a conductive portion 12 c which extends through the baseplate 11 to electrically connect the pad portion 12 a, 121 a or 122 aand the ball grid portion 12 b, 121 b or 122 b. (For the simplicity ofexplanation, description will be made from now on with reference to thepad portion 12 a, ball grid portion 12 b and conductive portion 12 c asrepresentative of the above mentioned pads and conductors, unless it isrequired to refer to other pads and conductors for particular purpose.)

[0050] The pad portions 12 a are arranged at a small pitch to adapt tothe pads connected to semiconductor chips, whereas the ball gridportions 12 b are arranged at a larger pitch as compared with the pitchof the pad portions 12 a. In this embodiment, the circuit board 10having the above construction is referred to as a work to be tested bythe apparatus. However, it is needless to say that a circuit board to betested by the apparatus is not limited to the above mentioned type. Itshould be noted that although the drawing shows only three wirings forclarification, actual circuited boards are formed with a great number ofwirings on the top and bottom surfaces or in the inside or both the twosurfaces and the inside of the circuit board.

[0051] The apparatus is provided with a work holder 21 to carry onepiece of a circuit board as a work 10. The work holder 21 is movablebetween a test position (position shown in FIG. 1) where the work 10 istested and a load/unload position (not shown) where the work 10 isloadable to the work holder 21 or unloadable from the work holder 21. Awork driving mechanism 22 drivingly reciprocate the work holder 21 backand forth between the test position and the load/unload position inaccordance with a control signal from a controller 30 which controls anoverall operation of the apparatus.

[0052] A lower fixture unit 40 is provided below the work 10 at the testposition. The lower fixture unit 40 includes a plurality of conductivespring probes 41 which are arranged in correspondence to the ball gridportions 12 b of the respective wrings 12. The lower fixture unit 40further include a multiplexer 42, and a lower fixture base (not shown)which is movable toward and away from the work 10 while carrying theprobes 41 and the multiplexer 42 thereon. The lower fixture base iscoupled to a lower fixture unit driving mechanism 43. The lower fixtureunit driving mechanism 43 drivingly moves the lower fixture base towardand away from the work 10 in response to a control signal from thecontroller 30.

[0053] An upper fixture unit 50 is provided above the work 10 at thetest position. The upper fixture unit 50 includes a cap-like housingwhich is so configured as to cover a certain region on one surface ofthe work 10. The housing includes a plate electrode 51 made of atransparent electrode and shielding members 52 made of, e.g., a rubber.The upper fixture unit 50 is movable toward and away from the work 10 asan integral unit. With this arrangement, when an upper fixture unitdriving mechanism 55 coupled to the upper fixture unit 50 is actuated inresponse to a drive command from the controller 30, the upper fixtureunit 50 is moved to the work 10. When an end portion 52 a of theshielding members 52 come into contact with the surface of the work 10the shielding member deforms and abuts against the surface of the work10 due to counter pressure. The end portion 52 a serves to make airtightthe enclosure surrounded by the housing 50 and the work 10. In thisembodiment, the end portion 52 a of the shielding member 52 itself isdeformable for the sealing of enclosure. However, this invention is notlimited to this arrangement. A seal member may be provided between theshielding member 52 and the work 10 as the case may be.

[0054] An atmosphere controller 70 is operatively connected to thehousing 50 to depressurize the closed space SP. When the work is tested,the closed space SP is preferably held at a vacuum degree of 10⁻² atm.In the case of a vacuum degree lower than 10⁻² atm, the electrondischarge rate decreases. On the other hands, higher vacuum degreeincreases the electron discharge rate. However, a longer time isrequired until the closed space SP reaches a desired higher vacuumdegree, consequently increasing the test time. According to experimentsof the inventors, it was confirmed that sufficient photo-electrons aredischarged under the pressure of 10⁻² atm which can be attained in arelatively short time.

[0055] The housing 50 may preferably have such a size as to cover theregion on the work 10 within which the pad portions 12 a of the wiringto be tested are located. With this arrangement, the closed space SPwhich needs depressurization can be minimized. As a result, theapparatus as a whole can be made small, and the time required fordepressurization can be shortened.

[0056] An electromagnetic wave irradiator 60 is provided in theapparatus to irradiate an electromagnetic wave onto one terminal or padof the wiring under test, i.e. selected one of a number of wirings 12formed on the work 12. The electromagnetic wave irradiator 60 includesan electromagnetic wave emitting section 61 which emits anelectromagnetic wave L in response to an operation command from thecontroller 30. An electromagnetic wave scanning section 62 directs theelectromagnetic wave L to a desired location on the work 10 in responseto an operation command from the controller 30. According to the presentembodiment, electromagnetic wave emitting section 61 is constructed soas to emit ultraviolet laser light beams having a wavelength of 266 nm.Also, the electromagnetic wave emitting section 61 is provided with anoptical system to focus the laser light beams on the pad portion 12 a ofa target wiring 12.

[0057] In this embodiment, the electromagnetic wave emitting section 61emits ultraviolet laser light beams for the purpose of causingphotoelectric effect. However, this invention is not limited to thearrangement of the embodiment, and visible light beams, infrared lightbeams or its equivalent may be used.

[0058] It is generally known that the photoelectric effect comes intobeing under the following condition:

Photon Energy≧Work Function Specific to Material:Energy for DischargingElectron from Material.

[0059] Thus, light energy should be given to the material to satisfy theabove mentioned inequality.

[0060] The electromagnetic wave emitting section 61 is so constructed asto be driven based on a pulse signal with use of a Q switching elementand the like. The electromagnetic wave scanning section 62 includes agalvanometer for the control of the direction of the electromagneticwave.

[0061] A direct current (DC) power source 80 is provided in theapparatus to apply a potential difference or a voltage between the plateelectrode 51 and the ball grid 12 b as the opposite terminal of thewiring under test. According to the present embodiment, the applicationof a voltage in the above manner enhances capturing of the electron bythe electrode 51 and enables efficient testing of the wiring by means ofphotoelectron caused by projection of the electromagnetic wave such as alaser beam while suppressing return and dispersion of dischargedelectrons and formation of a spatial space of electric charges as wellas was seen in the prior arts.

[0062] Further, a current detecting section 90 is provided in aconductive circuit pathway through which a current runs from oneterminal of the power source 80 to the opposite terminal thereof via theplate electrode 51 and the target wiring to detect the current runningin the circuit pathway. Specifically, the plus terminal of the powersource 80 is electrically connected to the plate electrode 51. The minusterminal of the power source 80 is connected to one terminal of themultiplexer 42 via the current detecting section 90, while the oppositeterminal of the multiplexer 42 is connected to a number of probes 41which are in contact with respective corresponding ball grid portions 12b of the wirings 12.

[0063] In this embodiment, when one wiring is selected from the numberof wirings 12 by switching over switch portions constituting themultiplexer 42 in response to a selection command from the controller 30with an output voltage from the power source 80 being applied betweenthe ball grid portion 12 b of the wiring under test and the plateelectrode 51.

[0064] Subsequently, the electromagnetic wave irradiator 60 irradiatesan electromagnetic wave L which, in turn, is projected onto the padportion 12 a of the target wiring, to thereby discharge electrons fromthe surface of the pad portion 12 a due to photoelectric effect. Thedischarged electrons are electrically attracted by the plate electrode51 aided by the voltage applied thereto. This arrangement eliminates alikelihood that electrons discharged once may be returned to the padportion or dispersed to the other pad portion(s), or form a spatialregion of electric charges, as occurred in the conventional arrangement.

[0065] In this embodiment, electrons are discharged from the surface ofthe pad portion 12 a which is connected to the ball grid portion 12 b.Accordingly, when the wiring under test is continuous and has noopen-circuit portion, a conductive circuit pathway is established fromthe plus terminal of the power source 80 to the minus terminal of thepower source 80 via the plate electrode 51, the target wiring 12, theprobe 41, the multiplexer 42, and the current detecting section 90. Thecurrent detecting section 90 measures a current running in the pathway,and outputs an analog signal corresponding to the measured current.Thus, in this embodiment, the plate electrode 51 serves as an electrodeportion, and the current detecting section 90 serves as a currentdetector.

[0066] In the embodiment mentioned above with reference to FIG. 1, one42 a of a number of switch portions constituting the multiplexer 42 isconnected to the opposite terminal of the power source 80, a voltage isapplied to the probe 41 a connected to the switch portion 42 a, and anelectromagnetic wave L is projected onto a pad portion 121 a connectedto the probe 41 a. In this case, a wiring 121 is a target wiring orwiring under test. If the wiring 121 is in a normal continuous state, acertain value of a current runs through the aforementioned conductivecircuit pathway due to the electrons discharged from the surface of thepad portion 121 a. On the other hand, if the wiring 121 is in adiscontinuity or open-circuited, a current value detected by the currentdetecting section 90 is zero or exceedingly lower than a current valuedetected in the case where the wiring 121 is in continuity. Thisarrangement enables the controller 30 to determine whether the targetwiring 121 is in continuity or in discontinuity based on the currentdetected by the current detecting section 90. Thus, in this embodiment,the controller 30 has a function of determining the state of the testedwiring as well as other various operation control function.

[0067] When an open circuit test with respect to the target wiring 121is completed, and the connection of the switch portions is switched overto another probe. When a new target wiring is selected by the switchportions and an electromagnetic wave is projected onto a pad portion ofthe new target wiring, an open circuit test of the new target wiring isperformed in the same manner as mentioned above. Thus, in thisembodiment, the multiplexer 42 serves as a selector.

[0068] In this embodiment, the switch connection by the multiplex 42 andselective irradiation of the pad portion also enables testing ofshort-circuit between a pair of wirings. Here, description is made for acase where the wiring 12 provided on the left side of the work 10 inFIG. 1 is referred to as “first target wiring”, and the wiring 121provided substantially transversely in the middle of the work 10 isreferred to as “second target wiring”, and a test is performed as towhether there is a short-circuited portion between the wiring pair 12and 121. In this case, for example, a middle switch portion 42 a whichis electrically connected to the second target wiring 121 is connectedto the opposite terminal of the power source 80. An electromagnetic waveL is irradiated onto the pad portion 12 a of the first target wiring 12.

[0069] Under the above mentioned condition, an electric field isgenerated between the plate electrode 51 and the pad portion 12 a (oneend) of the first target wiring 12 by application of a voltage to theplate electrode 51 and the second target wiring 121. Electronsdischarged by the irradiation of the laser beam, from the pad portion 12a of the first target wiring 12 are electrically attracted by the plateelectrode 51. In the case where a short-circuited portion exists betweenthe first target wiring 12 and the second target wiring 121, aconductive pathway is established through which a current runs from thepower source 80 and returns thereto via the plate electrode 51, thefirst target wiring 12, the short-circuited portion, and the secondtarget wiring 121. Thus, a current running through the target wiringpair 12 and 121 is measured by the current detecting section 90.

[0070] On the other hand, in the case where the target wiring pair 12and 121 is not in a short circuit state, the aforementioned conductivepathway is not established, and the current value detected by thecurrent detecting section 90 is zero or exceedingly lower than thecurrent value detected when the target wiring pair 12 and 121 is in ashort circuit state. This arrangement for the detection of a currentrunning in the target wiring pair enables efficient and stabledetermination whether the target wiring pair is in a short circuit stateor not. Test can be performed with respect to the other wiring pairs inthe similar manner as mentioned above. For instance, when the padportion of the wiring located on the right side of the work 10 in FIG. 1is irradiated with an electromagnetic wave L in the state shown in FIG.1, determination is made whether there is a short-circuited portionbetween the second target wiring 121 and the right-side wiring.

[0071]FIG. 3 is a flowchart showing an operation of the circuit boardtesting apparatus shown in FIG. 1. First, an untested work (circuitboard) 10 is loaded onto the work holder 21 at the load/unload positionby a handling device (not shown) provided in the testing apparatus or amanual operation by an operator (in Step S1). Then, the controller 30starts to control operations of the various parts of the apparatus toexecute the following steps S2 to S9 so as to test short andopen-circuit of the work 10.

[0072] First, at Step S2, the work holder 21 clamps the work 10. Thework holder 21 holding the work 10 thereon is moved to the test position(position shown in FIG. 1) where the work 10 is to be tested (Step S3).Thus, the work 10 is positioned at the test position.

[0073] Subsequently, the upper fixture unit 50 and the lower fixtureunit 40 are moved to the work 10 to pressingly hold the work 10therebetween (in Step S4). As shown in FIG. 1, when the lower fixtureunit 40 is moved to the work 10 at the test position, a lead end of eachof the conductive spring probes 41 is brought into pressing contact withthe corresponding ball grid portion 12 b of the wiring 12 to therebyelectrically connect the work 10 to the lower fixture unit 40.Simultaneously, as the upper fixture unit 50 is moved to the work 10 atthe test position, the housing 51 and the work 11 form an airtightclosed space SP as shown in FIG. 1.

[0074] Thus, when the apparatus is set up for testing the work 10, anopen circuit test (Step S5) and a short circuit test (Step S6) areimplemented to test a continuity of the wirings of the work 10. Thesetests will be described in detail later.

[0075] Upon completion of the tests, the lower fixture unit 40 and theupper fixture unit 50 are moved away from the work 10 to release thework 10 from the fixtures (in Step S7). The work holder 21 is moved tothe load/unload position to release clamping of the work 10 (in StepS8). At a final stage, upon verifying that the work 10 after the testshas been unloaded from the work holder 21 in Step S9, the routinereturns to Step S1 to execute the aforementioned series of operationswith another work.

[0076] Next, the open circuit test (Step S5) is described in detail withreference to FIGS. 4 and 5. FIG. 4 is a flowchart showing an opencircuit test to be implemented by the apparatus. FIG. 5 is a timingchart for the open circuit test.

[0077] The closed space SP secured in Step S4 is filled with aircontaining oxygen. If an electromagnetic wave is irradiated onto the padportion 12 a in the closed space SP in this state, it is highly likelythat molecules in the air obstruct electrons generated by photoelectriceffect from being properly discharged from the surface of the padportion, which makes it difficult to stably measure a current due to theelectrons. To avoid such a drawback, in this embodiment, the atmospherecontroller 70 is activated to depressurize the interior of the housing50 to approximately 10⁻² atm in response to an operation command fromthe controller 30 (in Step S51).

[0078] Upon completion of depressurization, as shown in FIG. 5, themultiplexer 42 is activated in accordance with a selection command fromthe controller 30, and one wiring 12 (target wiring) is electricallyconnected to the minus output terminal of the power source 80 (in StepS53). Thus, the first target wiring is selected with the voltage of thepower source 80 being applied between the electrode 51 and the ball gridof the selected wiring. Then, ultraviolet laser light in the form ofpulses or other type of electromagnetic wave is irradiated onto the padportion 12 a of the selected wiring at a predetermined timing shown inFIG. 5 (in Step S54).

[0079] During irradiation, the current detecting section 90 measures thecurrent which changes as shown in FIG. 5 (in Step S55). It is judgedwhether the target wiring is in an open circuit state or not based onthe measured current value (in Step S56). A judgment regarding an opencircuit can be performed merely based on presence or absence of adetected output. Preferably, however, an open circuit may be judged bycomparison of a current value measured with a reference circuit board,with a current measured with the circuit board under test. A series ofoperations from selection of the target wiring (Step S53) to judgmentregarding open circuit (Step S56) are repeated until it is judged thatall the wirings has been tested in Step S57.

[0080] As mentioned above, in the testing apparatus in accordance withthe first embodiment, an electric field is generated between the plateelectrode 51 and the pad portion 121 a by application of a voltage tothe plate electrode 51 and the ball grid portion (opposite terminal) 121b of the target wiring 121. Electrons which have been discharged fromone terminal of the target wiring 121 by photoelectric effect due toelectromagnetic wave irradiation are electrically attracted by the plateelectrode 51 aided by the existence of the electric field. With thisarrangement, in the case where the target wiring 121 is in continuity, aconductive pathway is established through which a current runs from thepower source 80 and returns thereto via the plate electrode 51 and thetarget wiring 121, and a current running through the target wiring 121can be stably measured by the current detector.

[0081] On the other hand, in the case where the target wiring 121 is indiscontinuity, the aforementioned conductive pathway is not established,and the current value detected by the current detecting section 90 iszero or exceedingly lower than the current value detected in the casewhere the target wiring 121 is in continuity. In this arrangement, therecan be determined precisely and stably whether the target wiring 121 isin continuity by detecting a current running through the target wiring121.

[0082] In this embodiment, the closed space SP enclosing the pad portionto be irradiated is depressurized, and molecules in the air inside theclosed space SP which are liable to hinder discharge of electronsgenerated by photoelectric effect can be reduced. Thereby, electrons areefficiently discharged, and a stable current measurement is enabled.Further, since the housing 50 defining the closed space SP is soconfigured as to cover a minimal area on the work 10, the space fordepressurization can be minimized, which contributes to production of asmall-sized apparatus and shortening of a time required fordepressurization.

[0083] In this embodiment, a conductive pathway through which a currentruns from the plus terminal of the power source 80 to the minus terminalof the power source 80 via the plate electrode 51, the target wiring 12,the probe 41, the multiplexer 42, and the current detecting section 90is established, and a judgment as to whether the target wiring is in anopen circuit state is made by measuring a change of current runningthrough the conductive pathway. In other words, since the testingapparatus is so constructed as to establish a conductive circuitpathway, a current value can be measured stably.

[0084] Further, in this embodiment, a transparent electrode is used asthe plate electrode 51. This arrangement is advantageous in thefollowing point. An electromagnetic wave can be irradiated onto the padportion of the target wiring even if the plate electrode 51 is providedhigh above the target wiring because the electromagnetic wave passesthrough the transparent electrode 51 and is irradiated onto the padportion. In view of the above, in this embodiment, the plate electrode51 can be disposed closer to the pad portion 121 a of the target wiring121, and electrons discharged from the pad portion 121 a uponirradiation can be securely trapped by the plate electrode 51 to therebysecure a more stable test.

[0085] Furthermore, in this embodiment, since the plate electrode 51 hassuch a shape as to cover a group of wirings to be tested, the followingeffects can be obtained. Specifically, this arrangement does not need totransversely move the plate electrode 51 to match with the location ofthe target wiring, and allows an electromagnetic wave to pass throughthe plate electrode 51 and irradiate onto the target wiring while fixingthe plate electrode 51. This arrangement enables to simplify theconstruction of the upper fixture unit 50 and the upper fixture unitdriving mechanism 55 and shorten a test time. Further, since the plateelectrode 51 constitutes a portion of the housing 50, the number ofparts constituting the apparatus can be lessened.

[0086] Next, the short circuit test (Step S6) is described withreference to FIG. 6. FIG. 6 is a flowchart showing a short circuit testby the apparatus. An overall flow of the short circuit test is basicallythe same as the open circuit test (Step S5) except that the shortcircuit test includes switch-over control of the multiplexer 42 inassociation with irradiation onto the pad portion. Hereinafter, merelythe differences between the short circuit test and the open circuit testare described primarily focusing on the short circuit test.

[0087] Similar to the open circuit test, in the short circuit test,after depressurization is performed (in Step S61), a pair of targetwirings are selected in accordance with a selection command from thecontroller 30 (in Step S63) with a voltage being applied between theelectrode 51 and one of the selected wiring (in Step S62). At this time,the multiplexer 42 is activated in response to a selection command fromthe controller 30 in such a manner that the minus output terminal of thepower source 80 is not electrically connected to the first target wiringconstituting the target wiring pair but is connected to the secondtarget wiring constituting the target wiring pair. On the other hand,the scanner 62 is controlled to direct the laser light beam to the pador end terminal of the first target wiring.

[0088] After the target wiring pair is selected in Step S63, anelectromagnetic wave is irradiated onto the pad portion of the firsttarget wiring in response to an operation command from the controller 30(in Step S64). Thereupon, electrons are discharged from the pad portion,and an electric field is generated between the plate electrode 51 andthe pad portion (one terminal) of the first target wiring by applicationof a voltage to the plate electrode 51 and the second target wiring ifthe target wiring pair is in a short circuit state. As a result, theelectrons discharged from the first target wiring due to photoelectriceffect by electromagnetic wave irradiation are electrically attracted bythe plate electrode 51 aided by the existence of the electric field, anda conductive pathway is established through which a current runs fromthe power 80 and returns thereto via the plate electrode 51, the firsttarget wiring, the short-circuited portion, and the second target wiringto thereby securely measure a current running through the target wiringpair.

[0089] On the other hand, in the case where the target wiring pair isnot in a short circuit state, the aforementioned conductive pathway isnot established, and a current value detected by the current detectingsection 90 is zero or exceedingly lower than a current value detected inthe case where the target wiring pair is in a short circuit state. Thus,this arrangement enables to precisely and stably determine whether thetarget wiring pair is in a short circuit state by detecting a currentrunning through the target wiring pair.

[0090] In this embodiment, during irradiation, the current detectingsection 90 measures a current and outputs a signal corresponding to thecurrent as a detected output (in Step S65). It is judged whether thetarget wiring pair is in a short circuit state based on the measuredcurrent value (in Step S66). A judgment regarding short circuit can beperformed simply based on presence or absence of a detected output.Preferably, however, it is judged whether the target wiring pair is in ashort circuit state by comparing a current value measured with areference circuit board with a current value measured with the circuitboard under. A series of operations from selection of the target wiringpair (Step S63) to judgment regarding short circuit (Step S66) arerepeated until it is judged that all the wirings on the work 10 has beentested in Step S67.

[0091] In the above mentioned first embodiment, a transparent electrodeis used as the plate electrode 51. This invention is not limited to thatarrangement. Alternatively, a mesh electrode may be provided in place ofthe plate electrode. In the altered arrangement, it is preferable that ahousing is made of a transparent glass material or the like and a meshelectrode is attached on an inner surface of the housing. In such analtered arrangement, an electromagnetic wave L passes through thehousing and clearances between the mesh electrodes to be irradiated ontoa target wiring. This altered arrangement enables to obtain a similareffect as the first embodiment.

[0092] Further, it would be appreciated to provide an electrode on theside of a housing defining a closed space SP instead of the provision ofan electrode in a top of the housing. Specifically, a side wall of thehousing may be made of conductive metallic material to function asshield and electrode while a top of the housing is made of transparentglass. This construction makes connection of the electrode with anexternal power source easier.

[0093] Next, a modification of the first embodiment is described. FIG. 7is a diagram showing the modified circuit board testing apparatus. Thebasic principle of the modified apparatus is similar to that of theapparatus in accordance with the first embodiment. The modificationdiffers from the first embodiment in the manner of applying a voltagefrom a power source and in the arrangement in association therewith. Inview of this, constituent elements in the modification which areidentical to those in the first embodiment are denoted by the samereference numerals, and the modification is described primarily focusingon the difference of the modification from the first embodiment.

[0094] The modified apparatus is not provided with a plate electrode forapplying a voltage. In the modified apparatus, a voltage is applied toall or part of wirings arranged in the vicinity of a target wiring suchthat the wirings may efficiently capture the electrons discharged fromthe target wiring upon irradiation of an electromagnetic wave. Toprovide this arrangement, in the modification, the plus terminal of apower source 80 is connected to one terminal of a multiplexer 45,whereas the minus terminal of the power source 80 is connected to theopposite terminal of the multiplexer 45 via a current detecting section90.

[0095] An upper fixture unit includes a housing 54 having the shape of acap to cover a certain area on one surface of a work 10. An opticalwindow is formed in the housing 54 at a position above a target wiring.The optical window constitutes an irradiation path for guiding anelectromagnetic wave L.

[0096] More specifically, the optical window may be formed through whichan electromagnetic wave L is irradiated, or the entirety of the housing54 is made of a glass which is optically transparent or its equivalent.The housing 54 constituting the upper fixture unit is movable toward andaway from the work 10. An upper fixture unit driving mechanism 55 isactivated in response to a drive command from a controller 30. Thehousing 54 is moved to the work 10 until its bottom edge 54 a of thehousing 54 comes into contact with a surface of the work. Then, the endportion or bottom edge 54 a is deformed and pressed against the surfaceof the work due to counter pressure. The end portion 52 a serves as atight closure or seal. In this way, an airtight closed space SP isdefined by the work 10 and the housing 54.

[0097] Described is a case, as shown in FIG. 7 for example, in which aswitch portion 45 a is connected to a terminal a, and switch portions 45b and 45 c which are remaining switch portions of the multiplexer 45 areconnected to a terminal b. In this case, a wiring 121 connected to theswitch portion 45 a is a target wiring. A certain level of voltage isapplied to wirings connected to the switch portions 45 b, 45 c from thepower source 80, and an electromagnetic wave L is irradiated onto a padportion 121 a.

[0098] In the case where the wiring 121 is in a normal continuous state,an electric field is generated between pad portions 12 a of the wiringsconnected to the switch portions 45 b, 45 c (hereinafter, referred to as“the other wirings”) and the pad portion 121 a of the target wiring 121by applying a voltage to the opposite terminal of the target wiring 121and the other wirings. Electrons which have been discharged from the padportion 121 a of the target wiring 121 by photoelectric effect due toelectromagnetic wave irradiation are electrically attracted by the padportions 12 a.

[0099] At this time, in the case where the target wiring 121 is incontinuity, a conductive pathway is established along which a currentruns from the power source 80 and returns thereto via the other wiringsand the target wiring 121 to thereby cause a current running through thetarget wiring 121, with the current being measured by the currentdetecting section 90.

[0100] On the other hand, in the case where the target wiring 121 is indiscontinuity, the aforementioned conductive pathway is not established,and a current value detected by the current detecting section 90 is zeroor exceedingly lower than a current value detected in the case where thewiring 121 is in continuity. This arrangement enables to precisely andstably determine whether the target wiring is in continuity by detectinga current running through the target wiring 121, and enables thecontroller 30 to determine whether the target wiring 121 is incontinuity or in discontinuity based on a measured current detected bythe current detecting section 90.

[0101] When an open circuit test with respect to the target wiring 121is completed, and the connecting state of the switch portions isswitched over, a new target wiring is selected one after another. Afterthe new target wiring is selected by switching over the switch portions,and an electromagnetic wave is irradiated onto a pad portion of the newtarget wiring, an open circuit test with respect to the new targetwiring can be performed in the same manner as mentioned above. Thus, theopen circuit test can be performed with respect to all the wirings ofthe work 10.

[0102] In the modification of the first embodiment, it is required toperform a short circuit test with respect to respective pairs of ballgrid portions prior to an open circuit test when the open circuit testis to be performed with use of the modified apparatus. This is becausein the case where there is a short-circuited portion between a pair ofball grid portions, it is highly likely that a current may erroneouslyrun when the switch portions of the multiplexer are about to be switchedover. Such short circuit testing may be made, for example, by connectingone terminal of the power source to one of the wiring and the otherterminal of the power source to another wiring through a currentmeasuring device, without the above mentioned irradiation ofelectromagnetic wave.

[0103] As mentioned above, in the modification, the pad portions 12 a ofthe other wirings serve as the plate electrode in the first embodimentby selectively switching over the switch portions of the multiplexer 45.While generating an electric field between the pad portion 121 a of thetarget wiring 121 and the pad portion(s) 12 a of the other wiring(s),electrons generated from the pad portion 121 a due to photoelectriceffect by electromagnetic wave irradiation are trapped by the padportion 121 a. With the arrangement of this modification,continuity/discontinuity of the target wiring can be stably determinedin the similar manner as in the first embodiment despite the fact that aplate electrode is not provided in the modification.

[0104] This invention is not limited to the aforementioned firstembodiment and the modification thereof. For instance, in the apparatusin accordance with the first embodiment (or the modification), an opencircuit test and a short circuit test are performed in this order todetermine whether a work (circuit board) 10 is in continuity. The orderof testing is not limited to the above. Further, this invention isapplicable to any apparatus as far as the apparatus is capable ofperforming at least an open circuit test.

[0105] In the first embodiment and the modification thereof, the circuitboard 10 capable of mounting a semiconductor chip according to C4package method is used as a work to be tested. Alternatively, thisinvention is applicable to test a circuit board in which one surface ofa base plate is formed with wirings or a circuit board formed with acuffed wiring pattern.

[0106] In the first embodiment and the modification, an electromagneticwave L is irradiated in the form of a pulse for one time. The number oftimes of irradiation is not limited to one, and the irradiation may beperformed for a certain number of times. Further, in the firstembodiment and the modification, depressurization of the interior of ahousing is performed. Alternatively, as the case may be,depressurization may be omitted, or vacuum degree may be varieddepending on performance of the electromagnetic wave irradiator.

[0107] As mentioned above, according to the first embodiment and themodification, an electric field is generated between an electrodeportion and one terminal of a target wiring, and a conductive pathway isestablished by attracting electrons discharged from the one terminal ofthe target wiring by photoelectric effect due to electromagnetic waveirradiation onto the electrode portion aided by the existence of theelectric field. Thereby, short and open-circuit of the target wiring canbe accurately and stably tested.

[0108]FIG. 8 is a schematic illustration of a circuit board testingapparatus according to a second embodiment of the invention. FIG. 9 is ablock diagram showing an electric configuration of the testing apparatusin FIG. 8. A circuit board testing apparatus in accordance with a secondembodiment is adapted to test a circuit board 210. As shown in FIG. 8,the circuit board 210 is constructed in such a manner that a number ofwirings 212,321 and 322 are formed on a base plate 211. It is to beappreciated that actual circuit board or substrate has many wiringsformed thereon but that only three wirings are shown in the drawing.Description will be made herein after with reference to the wiring 210as a representative of other wirings, for the convenience unless otherwirings are required to be referred to for particular explanation.

[0109] Terminals 212 a and 212 b of the wiring 212 are formed on thecircuit board 210 or substrate to be connected with an electroniccomponent mounted on the circuit board 210 or external wirings. Aconductive portion 212 c is formed on the surface of or inside thecircuit board 210 to electrically connect the terminals 212 a and 212 b.In this embodiment, described is a case where the circuit board 210having the above construction is tested as a work. It is needless to saythat the work to be tested by this embodiment is not limited to theaforementioned circuit board. In this embodiment, the terminals 212 aand 212 b are provided on the respective surfaces of the circuit board210, and the conductive portion 212 c which connects the terminals 212 aand 212 b is provided inside the base plate 211. Alternatively,terminals may be formed on either one of the surfaces of the circuitboard, and a conductive portion for connecting the terminals may beformed on the same or opposite side surface of the circuit board.

[0110] The testing apparatus includes a lower fixture unit 240 which isprovided with a holding section for holding a circuit board 210 as awork thereon. The lower fixture unit 240 includes a metallic plate 241,an insulating film 242 formed on the upper surface of the metallic plate241, and a lower fixture base 245 which integrally holds the metallicplate 241 and the insulating film 242 thereon. The metallic plate 241has such a dimension as to substantially cover the lower surface of thework 210 in order to maximize a capacity provided by a wiring 212 formedon the work 210 and the metallic plate 241. The metallic plate 241 iscoated with an insulating film 242 on an upper surface thereof. Withthis arrangement, when the circuit board 210 is placed on the metallicplate 241, terminals 212 b formed on the lower surface of the circuitboard 210 are reliably kept from coming into contact with the metallicplate. The lower fixture base 245 is coupled to a lower fixture drivingmechanism 246. The lower fixture unit driving mechanism 246 drivinglyreciprocate the lower fixture unit 240 back and forth between a testposition (position shown in FIG. 8) where the work 210 is tested and aload/unload position (not shown) where the work is loaded on andunloaded from the lower fixture unit 240.

[0111] A conductive probe 281 is provided at the test position. When thelower fixture unit 240 is moved to the test position, the metallic plate241 provided on the lower fixture unit 240 is rendered into contact withthe conductive probe 281. Thus, the metallic plate 241 is electricallycommunicable with a power source 70 which is described later.

[0112] It should be noted that the insulating film 242 is not a materialelement. The metallic plate 241 is not required to be coated with theinsulating film 242 in the case of the apparatus being applied for acircuit board formed with a wiring pattern only on a top surface or acircuit board formed with an insulating layer over wiring patterns. Inthat case, the metallic plate 241 may be in direct contact with such acircuit board without the insulating film 242. Also, even if the work210 is a circuit board formed with a wiring pattern on both surfacesthereof, as will be described later, a test may be performed for such acircuit board by an apparatus which is not provided with an insulatingfilm.

[0113] An upper fixture unit 250 is arranged above the work 210. Theupper fixture unit 250 is provided with a housing 251 in the form of acap so as to cover terminals 212 a, 321 a, 321 aa and 322 a formed onthe upper surface of the work 210. The housing 251 is formed with anexhaust port 254 on a side wall thereof, and is made of, e.g., atransparent silica glass. Also, the housing 251 is provided with a sealmember 252 made of, e.g., rubber on a free end of a side wall thehousing 251. Further, a transparent plate electrode 253 is attached ordeposited on an inner upper surface of the housing 251.

[0114] Further, the side wall of the housing 251 may be formed by ametallic material with its top wall being formed by a transparent glass.In this case, the metallic side wall may be used as electrode. A unitcomprised of these constituent elements 251 through 254 is operativelyconnected with an upper fixture driving mechanism 256, and is movabletoward and away from the work 210.

[0115] The upper fixture unit 250 is moved to the work 210 until theseal member 252 on the end portion of the side wall of the housing 251comes into contact with the surface of the work 210. The seal member 252is resiliently deformed against the surface of the work 210. As aresult, an airtight enclosure or closed space SP is defined by the work210, the seal member 252 and the housing 251.

[0116] The exhaust port 254 formed on the housing 251 is communicatedwith an exhausting device 290 via an exhaust pipe (not shown). When theexhausting device 90 is activated based on a control signal from thecontroller 201, the air inside the closed space SP is drawn out todepressurize the interior of the closed space SP to about 10⁻² atm.

[0117] It is preferable to hold the closed space SP at a vacuum degreeof about 10⁻² atm when a test is performed. In the case of a vacuumdegree lower than 10⁻² atm, the electron discharge rate decreases. Onthe other hands, higher vacuum degree increases the electron dischargerate. However, a longer time is required until the closed space SPreaches a desired higher vacuum degree, consequently increasing the testtime. According to experiments of the inventors of the presentinvention, it was confirmed that a sufficient amount of electrons aredischarged under the pressure of 10⁻² atm which can be attained in arelatively short time.

[0118] An electromagnetic wave irradiator 260 is provided in theapparatus to irradiate an electromagnetic wave to a terminal connectedto one wiring (target wiring) alternatively selected from a plurality ofwirings 12 for the test. The electromagnetic wave irradiator 260includes an electromagnetic wave emitting section 261 which emits anelectromagnetic wave L in response to an operation command from thecontroller 201. An electromagnetic wave scanning section 262 directs theelectromagnetic wave L to a desired location on the work 210 in responseto an operation command from the controller 201.

[0119] The electromagnetic wave emitting section 261 is constructed soas to emit ultraviolet laser light beams having a wavelength of 266 nm.Also, the electromagnetic wave emitting section 261 is provided with anoptical system to focus the laser light beams on the pad portion 212 aof a target wiring 212.

[0120] In this embodiment, the electromagnetic wave emitting section 261emits ultraviolet laser light beams for the purpose of causingphotoelectric effect. However, this invention is not limited to thearrangement of the embodiment, and visible light beams, infrared lightbeams or its equivalent may be used. The electromagnetic wave emittingsection 261 is so constructed as to be driven based on a pulse signalwith use of a Q switching element and the like. The electromagnetic wavescanning section 262 includes a galvanometer for changing the angle of amirror for directing the laser light beam. The electromagnetic waveirradiator 260 in accordance with this embodiment is constructed in sucha manner that the galvanometer is driven based on an operation commandfrom the controller 201 to project the electromagnetic wave L onto adesired location on the surface of the work 210 accurately and speedily.

[0121] A direct current power source 270 is provided in the apparatus toapply an electric potential difference or voltage between the plateelectrode 253 and the metallic plate 241. The DC power source 270outputs a certain voltage.

[0122] Further, a current detecting section 280 is provided at aposition in a conductive circuit pathway through which a current runsfrom one terminal of the power source 270 to the opposite terminalthereof via the plate electrode 253, a target wiring, and the capacitivecoupling of the metallic plate 241 and the target wiring to detect thecurrent running in the conductive pathway. Specifically, the plusterminal of the power source 270 is electrically connected to the plateelectrode 253, and the minus terminal of the power source 270 isconnected to the conductive probe 281 via the current detecting section280. The conductive probe 281 is in contact with the metallic plate 241when the lower fixture unit 240 and the work 210 are set at the testposition. Thus, the aforementioned conductive pathway is established.

[0123] In this embodiment, an electric field of which electric potentialis higher at the plate electrode 253 than at the metallic plate 241 isgenerated when the power source 270 applies a voltage between the plateelectrode 253 and the metallic plate 241. When an electromagnetic wave Lis irradiated onto the terminal 212 a of the wiring 212 in this state,electrons are discharged from the terminal 212 a due to photoelectriceffect. The electrons discharged from the terminal 212 a areelectrically attracted by the plate electrode 253 aided by the existenceof the electric field.

[0124] Further, in this embodiment, since a certain capacity is securedby the wiring 212 and the metallic plate 241, the following effect isobtained. When electrons discharged from the wiring 212 due tophotoelectric effect are trapped by the plate electrode 253 and traveltoward the plus terminal of the power source 270, the same amount ofelectrons as the discharged electrons run through the metallic plate 241from the minus terminal of the power source 270 via the currentdetecting section 280 and the conductive probe 281. Thus, a conductivepathway along which a current runs from the plus terminal of the powersource 270 and returns thereto via the plate electrode 253, the wiring212, the metallic plate 241, the conductive probe 281, and the currentdetecting section 280 is established, and the current running throughthe conductive pathway is detected by the current detecting section 280.The current value detected by the current detecting section 280 isconverted into a digital signal by an A/D converter circuit 281 and sentto the controller 201. In this embodiment, the plate electrode 253, themetallic plate 241, and the current detecting section 280 respectivelyserve as a first electrode portion, a second electrode portion, and acurrent detector.

[0125] In this embodiment, the current detecting section 280 is providedbetween the minus terminal of the power source 270 and the conductiveprobe 281. Alternatively, as far as a current running through theaforementioned conductive pathway is detectable, the current detectingsection may be provided, for example, between the plus terminal of thepower source 270 and the plate electrode 253.

[0126] Described is a case where an electromagnetic wave L is irradiatedonto a terminal 321 a of a wiring 321 as shown, e.g., in FIG. 8. In thiscase, the wiring 321 is a target wiring to be tested. When the targetwiring 321 is in a normal continuous state, the wiring 321 and themetallic plate 241 constitute a capacitor in which the terminals 321 a,321 aa, 322 b, and 321 c constitute an electrode having one polaritywhile the metallic plate 241 constitutes an electrode having theopposite polarity.

[0127] When an electromagnetic wave L is irradiated onto the terminal321 a, electrons are discharged from the terminal 321 a due tophotoelectric effect. The discharged electrons are electricallyattracted and trapped by the plate electrode 253 and run to the plusterminal of the power source 270. As a result of the electron discharge,the wiring 321 is charged positively. On the other hand, the oppositeelectrode of the capacitor, namely, the metallic plate 241 is chargednegatively with electrons being supplied from the minus terminal of thepower source 270. In this way, a current runs through the aforementionedconductive pathway due to irradiation of an electromagnetic wave ontothe terminal 321 a. Thus, the capacitor comprised of the wiring 321 andthe metallic plate 241 is charged.

[0128] The waveforms a in respective graphs of FIG. 10 show changes of apotential at the wiring 321, a current running through the currentdetecting section 280 and detected thereby, and an amount of electriccharges as an integration of the currents running through the currentdetecting section 280 while the electromagnetic wave is beingirradiated. Specifically, the diagrams in FIG. 10 are graphs showingchanges of a potential at a target wiring, a current running through theaforementioned conductive pathway, and an amount of electric chargeswhich have been charged at the capacitor, before irradiation ofelectromagnetic wave and while the electromagnetic wave is beingradiated. When irradiation of an electromagnetic wave L is initiated,electrons discharged from the terminal 321 a are electrically attractedtoward the plate electrode 253 and a current runs through theaforementioned conductive pathway. As electrons are discharged from thewiring 321, the potential of the wiring 321 is raised. As a result, thecurrent running through the conductive pathway is gradually decreased.When the potential of the wiring 321 reaches the same level as the plateelectrode 253, the electrons discharged from the terminal 321 a are nolonger electrically attracted toward the plate electrode 253, andrunning of current through the conductive pathway ceases. At this time,if it is assumed that a charged amount charged at the capacitor is Q₀,the capacity of the capacitor comprised of the wiring 321 and themetallic plate 241 is C₀, and an output voltage of the power source 270is V, the following equation is established:

Q ₀ =C ₀ ·V

[0129] On the other hand, in the case where the wiring 321 is in an opencircuit state, e.g., where there is an open circuit portion at point xin FIG. 8, one electrode of the capacitor covering the areacorresponding to the terminal 321 a and a portion of the conductiveportion 321 c extending up to point x, is smaller than the electrode ofthe capacitor formed by the entire length of the wiring 321 in a normalcontinuous state. As a result, the capacity of the capacitor in the opencircuit state is smaller than the reference capacity C₀ of the capacitorin the aforementioned normal continuous state. When the terminal 321 ais irradiated with an electromagnetic wave L in the open circuit state,changes of a potential at the wiring 321, a current detected by thecurrent detecting section 280, and a charged amount charged at thecapacitor for a time upon irradiation of electromagnetic wave are, forexample, as shown by respective waveforms b in the graphs of FIG. 10.

[0130] In the case where the wiring 321 is in a short circuit state,e.g., where there is a short-circuited portion at point y between thetarget wiring 321 and the other wiring 322 in FIG. 8, the wiring 321 andthe other wiring 322 constitute one electrode of the capacitor, and thecapacitance of the capacitor in the short circuit state is larger thanthe reference capacity Co of the wiring 321 in the aforementioned normalcontinuous state. When the terminal 321 a is irradiated with anelectromagnetic wave L in the short circuit state, changes of apotential at the wiring 321, a current detected by the current detectingsection 280, and an amount of electric charges that have been charged atthe capacitor for a time upon irradiation of electromagnetic wave are,for example, as shown by respective waveforms c in the graphs of FIG.10. In the case where the target wiring is in an open circuit state, theamount of electric charges corresponding to integration of the currentsthat have run through the current detecting section 280 is smaller thanthe reference charged amount Q₀, whereas in the case where the targetwiring is short-circuited with other wiring, the amount of electriccharges corresponding to integration of the currents that have runthrough the current detecting section 280 is larger than the referencecharged amount Q₀.

[0131] The controller 201 calculates the amount of the electricity Qactually charged at the capacitor which is calculated by integratingcurrent values measured by the current detecting section 280 whileelectromagnetic wave is being irradiated. Then, the controllerdetermines the continuity of the wiring 321 by comparing the actuallycharged amount Q with the reference charged amount Q₀ which iscalculated in advance with the wiring 321 at a normal continuous state.In this embodiment, the controller 201 has a function of determinator.

[0132] Next, an operation of the circuit board testing apparatus inaccordance with the second embodiment is described with reference toFIG. 11. FIG. 11 is a flowchart showing operations of the testingapparatus shown in FIG. 8. When an untested work (circuit board) 210 isloaded on the lower fixture unit 240 positioned at a load/unloadposition by a handling device (not shown) incorporated in the testingapparatus or a manual operation by an operator (in Step T1), thecontroller 201 start to control operations of the various parts of theapparatus to execute the following steps T2 to T11 so as to test shortsand opens-circuit of the work 210.

[0133] When the work 210 is loaded on the lower fixture unit 240, thelower fixture unit 240 is moved to the test position while carrying thework 210 thereon (in Step T2). Thus, the work 210 is positioned at thetest position. Then, the metallic plate 241 is brought into contact withthe conductive probe 281 to be connected to the current detectingsection 280.

[0134] Subsequently, the upper fixture unit 250 is moved to the work210, and fixedly sandwiches the work 210 between the upper fixture unit250 and the lower fixture unit 240 (in Step T3). As a result, anairtight closed space SP is defined by the housing 251, the seal member252 and the work 210. Then, the exhausting device 290 is activated todepressurize the interior of the closed space SP to a predeterminedpressure of about 10⁻² atm (in Step T4). The power source 270 outputs acertain DC voltage to be applied between the plate electrode 253 and themetallic plate 241 (in Step T5).

[0135] Thus, when the apparatus is set up for testing the work 210, atest as to whether a target wiring is in a normal continuous state isimplemented (in Step T6). The contents of the wiring test are describedin detail later.

[0136] Upon completion of the wiring test, the power source 270 suspendsits voltage output (in Step T7). After suspending activation of theexhausting device 290, the air outside the apparatus is drawn into theclosed space SP (in Step T8), and the upper fixture unit 250 isretracted away from the work 210 (in Step T9), and the lower fixtureunit 240 is moved to the load/unload position (in Step T10). At a finalstage, upon verifying that the work 210 after the wiring test has beenunloaded in Step T11, the routine returns to Step T1 to execute theaforementioned series of operations.

[0137] Next, the wiring test (Step T6) is described in detail withreference to FIG. 12. FIG. 12 is a flowchart showing procedures of thewiring test to be implemented by the apparatus.

[0138] When the routine is progressed to Step T5, the closed space SPdefined by the housing 251 and the work 210 has been depressurized to apredetermined pressure of about 10⁻² atm. In this state, the controller201 controls the operative angle of the galvanometer 262 so that laserbeams is focused on the terminal 321 a of a target wiring 321 (in StepT61). The laser beam emitted from the electromagnetic wave irradiator260 is an ultraviolet laser beam having a wavelength of 266 nm.Electrons discharged from the terminal 321 a due to photoelectric effectare electrically attracted by the plate electrode 253 aided by theexistence of the electric field, and a current runs through theconductive pathway. The current is measured by the current detectingsection 280 (in Step T62). The current measurement is continued for atime (in Step T63). Next, the controller 201 calculates a charged amountof electricity based on the current values detected by the currentdetecting section 280 (in Step T64). Specifically, the charged amount Qis calculated by integrating the measured current values on time-basis.Then, the controller 201 determines whether the target wiring 321 is ina normal continuous state or other state based on the calculated chargedamount Q (in Step T65).

[0139] In other words, in the case where the charged amount Q which hasbeen measured by actual measurement falls in a predetermined allowablerange including the predetermined reference charged amount Q₀ which hasbeen calculated in advance with respect to a wiring in a normalcontinuous state as a mean value, it is judged that the target wiring321 is in a normal continuous state. If the charged amount Q is lowerthan a lower limit of the predetermined allowable range, it is judgedthat the wiring 321 is in an open circuit state. If the charged amount Qexceeds an upper limit of the predetermined allowable range, it isjudged that the wiring 321 is in a short circuit state with respect tothe other wiring.

[0140] Thus, a test with respect to a target wiring is completed. Inthis way, the aforementioned series of operations with respect to awiring test is repeated with other wirings until the test is completedwith all the wirings of the work 210 (in Step T66).

[0141] As mentioned above, in the apparatus in accordance with thesecond embodiment, the metallic plate 241 provides a capacitive couplingof the metallic plate 241 with the wiring or wirings to be tested. Thecapacity provided by the capacitive coupling of the metallic plate 241and the target wiring varies depending on whether the target wiring isin continuity or in other state. Accordingly, the amount of electricitycharged at the capacitor comprised of the metallic plate 241 and thetarget wiring varies as the capacity varies. According to the secondembodiment, currents running through the predetermined conductivepathway via the capacitor are detected, an amount of electric chargesthat have been charged at the capacitor is calculated, and it is judgedwhether the target wiring is in a short circuit state or an open circuitstate based on the calculated charged amount. This arrangement enablesprecise and stable test of the wirings formed on a circuit board in acontactless manner.

[0142] As an alternative of the second embodiment, the testing apparatusis applicable to testing of continuity between two terminals or twowirings by changing a part of the aforementioned test procedures. FIG.13 is a flowchart showing steps of the altered test executable by thetesting apparatus in accordance with the second embodiment. FIGS. 14Aand 14B are sets of graphs each set showing changes of a potential atfirst and second terminals when the first and second terminals areirradiated with an electromagnetic wave, a current running through thecurrent detecting section 280 when the first and second terminals areirradiated, and an amount of electricity that have been charged at acapacitor for a time while electromagnetic wave is being irradiated andwith the irradiation of electromagnetic wave being switched over fromthe first terminal to the second terminal.

[0143] Since the arrangement of the testing apparatus for implementingthe altered test is identical to that of the testing apparatus inaccordance with the second embodiment, steps of the altered test aredescribed with reference to the flowchart of FIG. 13.

[0144] In this altered test, a terminal (first terminal) 321 a, forexample, is selected, and an electromagnetic wave L is irradiated ontothe selected terminal 321 a (in Step T611). Upon lapse of a time afterirradiation of the electromagnetic wave L, for example, at a timing t1(in Step T612), irradiation of the electromagnetic wave L is switchedover to a second terminal, for example, to a terminal 321 aa shown inFIG. 8 (in Step T613). At the same time, current values due toirradiation onto the first terminal 321 a and the second terminal 321 aafor respective times are measured (in Step T614), and the measuredcurrent values are integrated to calculate charged amounts with respectto irradiation onto the first terminal 321 a and the second terminal 321aa for the respective times (in Step T615). At this stage, if the firstterminal 321 a and the second terminal 321 aa are not in continuity, theelectrons discharged from the first terminal 321 a due to irradiation ofelectromagnetic wave onto the first terminal 321 a in Step T611 runtoward the plate electrode 253 at a high potential, whereby a currentruns through the plate electrode 253 along with potential rise of thefirst terminal 321 a. Thereafter, when irradiation of an electromagneticwave is switched over to the second terminal 321 aa in Step T613,electrons run from the second terminal 321 aa which is in a lowpotential toward the plate electrode 253 which is in a high potential.Changes of the potentials at the first terminal 321 a and the secondterminal 321 aa, the currents detected by the current detecting section280, and the charged amounts calculated by integrating the detectedcurrent values with respect to the first terminal 321 a and the secondterminal 321 aa in this state are, for example, as shown in respectivegraphs of FIG. 14A.

[0145] On the other hand, in the case where the potential of the firstterminal 321 a is raised by irradiation of electromagnetic wave onto thefirst terminal 321 a in Step T611 when the first terminal 321 a and thesecond terminal 321 aa are in continuity, the potential of the secondterminal 321 aa which is in continuity with respect to the firstterminal 321 a is also raised. In this state, even if the secondterminal 321 aa is irradiated with an electromagnetic wave in Step T613,electrons discharged from the second terminal 321 aa are notelectrically attracted toward the plate electrode 253, and an electricfield resulting from attraction of electrons is not generated. As aresult, the electrons do not travel toward the plate electrode 253, andthe current value detected by the current detecting section 280 is zeroor exceedingly lower than the current value detected in the case wherethe first terminal 321 a and the second terminal 321 aa are incontinuity. The potentials at the first terminal 321 a and the secondterminal 321 aa, the currents detected by the current detecting section280 with respect to the first terminal 321 a and the second terminal 321aa, and the charged amounts obtained by integrating the detectedcurrents when irradiation is switched over from the first terminal 321 ato the second terminal 321 aa in this state are, for example, as shownin respective graphs of FIG. 14B.

[0146] Upon completion of current measurements, the controller 201calculates a change of the charged amount Q on time-basis by integratingthe current values detected by the current detecting section 280 (inStep T616), and determines whether the first terminal 321 a and thesecond terminal 321 aa are in continuity based on the result ofcalculation (in Step T617). Specifically, in the case where the actuallymeasured charged amount Q varies before and after the timing t1, asshown in FIG. 14A, it is judged that the first terminal 321 a and thesecond terminal 321 aa are not connected with each other. On the otherhand, in the case where the changed amount Q does not vary before andafter the timing t1, as shown in FIG. 14B, it is judged that the firstterminal 321 a and the second terminal 321 aa are connected with eachother. Thus, a test with respect to one target wiring is completed. Theaforementioned series of operations with respect to the wiring test arerepeated until the test is completed with respect to all the wirings ofthe work 210 (in Step T618).

[0147] In the above embodiment, described is the case where the test isperformed between the first terminal 321 a and the second terminal 321aa which are designed to be continuous with each other as shown in FIG.8. In this case, if the terminals 321 a and 321 aa are in continuity, itis judged that the wiring test between the terminals 321 a and 321 aahas “PASSED”, whereas if the terminals 321 a and 321 aa arediscontinuous, it is judged that the terminals 321 a and 321 aa are inan open circuit state.

[0148] On the other hand, in the case where the test is performed byselecting terminals which are not designed to be continuous, e.g., inthe case of the terminals 321 a and 322 a, if the terminals 321 a and322 a are in discontinuity, it is judged that the wiring test betweenthe terminals 321 a and 322 a has “PASSED”, whereas if the terminals 321a and 322 a are in continuity, it is judged that the terminals 321 a and322 a are in a short circuit state. Thus, in the testing apparatus inaccordance with the second embodiment, a judgment as to whether anarbitrary combination of terminals of wirings formed on a circuit boardis in continuity or discontinuity enables to test opens and shorts ofthe wirings.

[0149] As mentioned above, in the second embodiment, an electromagneticwave is selectively irradiated on a plurality of terminals one afteranother, and it is judged whether the selected pair of terminals is incontinuity or not based on a change of an amount of electric chargesthat have run through and detected by the current detecting section 280before and after the irradiation is switched over between the pairs ofterminals. In this embodiment, a high potential is applied to the plateelectrode 253 which is provided in the vicinity of the terminals tosecurely allow the plate electrode 253 to trap electrons discharged fromthe terminals. This arrangement enables to precisely and stably testopens and shorts-circuit of the selected pairs of terminals.

[0150] In this embodiment, measured is a current that has run throughthe current detecting section 280 with the irradiation being switchedover from the first terminal to the second terminal. Alternatively, itmay be possible to allow the current detecting section 280 to keep onmeasuring a current for some time before the selected first terminal isirradiated so as to integrate the charged amount.

[0151] In this embodiment, it is required to monitor the current for atime period from the start of current flow until the current flow issuspended so as to calculate a charged amount Q for the monitored timeby integrating the monitored current values. In view of this, thisembodiment adopts a technique of securely detecting travel of electriccharges by continuously measuring currents for a time being whileelectromagnetic wave is being irradiated. Alternatively, change ofcurrents may be monitored until the current falls down to apredetermined level, current may be measured continuously until thecurrent or its integration becomes lowered than a predetermined value.

[0152] In the second embodiment, the amount of charge Q is calculated byintegrating current values on time-basis to judge whether the targetwiring is in continuity based on the calculated charged amount Q.Alternatively, a peak value of current may be detected to judge whetherthe detected peak value is lower than a reference value or to count atime until the detected current falls to a predetermined level so as todetermine whether the target wiring is in continuity.

[0153] Alternatively, a test may be performed by combining the test madein the second embodiment and any of the above mentioned alternativetests. As an example, the following arrangement is appreciated. Anelectromagnetic wave is irradiated onto the first terminal to perform atest with respect to a wiring (target wiring) connected to the firstterminal. When the target wiring is judged to be in an open or shortcircuit state, it is subsequently tested whether the target wiringrelative to the other wiring are in continuity. This arrangement enablesdetection of the portion and nature of defect of the tested circuitboard.

[0154] As mentioned above, there is a case that a test by a testingapparatus which is not provided with an insulating film 242 isadvantageous even if a work 210 is a circuit board having wirings on theopposite sides thereof. This is because a wiring 212 connected to aterminal 212 b functions as part of the second electrode portion byallowing the terminal 212 b which is formed on the lower surface of thecircuit board, to be electrically connected to a metallic plate 241 inthe case where such a circuit board is rendered into direct contact withthe metallic plate 241. In view of this, in the case where the work 210is, for example, a circuit board having a lower surface formed with aground layer, or a circuit board in which a terminal to be connected toa ground layer formed inside the circuit board is provided on the lowersurface of the circuit board, it is possible to function the groundlayer as part of the second electrode portion by rendering the work 210into direct contact with the metallic plate 241. At this time, thecapacity of the capacitor comprised of the target wiring and the secondelectrode portion can be raised, and the current running through thecapacitor can be increased with the result that detection of the currentby the current detecting section 280 is facilitated.

[0155] Further, since the position of the target wiring relative to thesecond electrode portion is clearly determined, a variation of capacityof the capacitor comprised of the target wiring and the second electrodeportion is lessened. As a result, a precise and stable test can beperformed.

[0156] In the second embodiment, providing the metallic plate 241 on thelower fixture unit 240 to oppose the metallic plate 241 to the work 210and connecting the metallic plate 241 to the power source 270 enables tofunction the metallic plate 241 as the second electrode portion. Forexample, in the case where the work 210 is a multi-layered substrate inwhich each of a plurality of layers formed with a wiring pattern areplaced one over another, it is impossible to secure a sufficientcapacity between a target wiring and the metallic plate 241 because theother wirings, a power source, or a ground layer may intervene betweenthe target wiring and the metallic plate 241. As a result, it is highlylikely that a precise and stable test cannot be performed. In such acase, functioning the wiring formed in the circuit board, e.g., theground layer as the second electrode portion enables to perform a wiringtest precisely and stably.

[0157]FIG. 15 is a diagram showing a testing apparatus as a firstmodification of the second embodiment in which a ground layer formed ina circuit board functions as the second electrode portion.

[0158] The testing apparatus as the first modification is adapted totest electric state of a circuit board 220. As shown in FIG. 15, thecircuit board 220 is formed with a plurality of wirings 222 on a baseplate 221. Each wiring 222 includes terminals 222 a and 222 b which areformed on the respective opposite surfaces of the circuit board 220, anda conductive portion 222 c which is formed on the surface or inside thecircuit board 220 and is electrically connected to the terminals 222 aand 222 b. A ground layer 223 is provided inside the base plate 221 toapply a reference potential to an electronic circuit established on thecircuit board 220 to implement predetermined operations of theapparatus. The ground layer 223 extends substantially over the entiresurface of the circuit board 220 except portions allowing passage of theconductive portions such as 222 c, and is connected to a terminal 223 awhich is formed on the upper surface of the circuit board 220 so as tobe electrically connected to an external ground. In this modification,described is a case where the circuit board 220 having the aboveconstruction is used as a work to be tested by the testing apparatus ofthe first modification. It is needless to say that the work to be testedby the apparatus is not limited to the aforementioned circuit board. Theinventive apparatus may test a circuit board, for example, in which aground layer 23 is a conductive member in the form of a mesh.

[0159] In this modification, a lower fixture unit 240 includes anon-conductive support block 243, whereas in the arrangement of thesecond embodiment, the lower fixture unit 240 includes the metallicplate 241 and the insulating film 242 as shown in FIG. 8. Thearrangement of the modification is advantageous in that the modificationdoes not require an electrode having a large surface area in the lowerfixture unit 240 since the ground layer 223 formed inside the circuitboard 220 serves as the second electrode portion. It should beappreciated that the arrangement of the second embodiment shown in FIG.8 also enables to perform the same test as in this modification.

[0160] Similar to the testing apparatus in accordance with the secondembodiment, the modified apparatus is constructed in such a manner thatan upper fixture unit 250 is moved toward the work 220 to securely holdthe work 220 between the upper fixture unit 250 and the lower fixtureunit 240, whereby an airtight closed space SP is defined by a housing251, seal member 252 and the work 220. The housing 251 is so constructedas to expose the terminal 223 a connected to the ground layer 223,outside the closed space SP. A conductive probe 257 is provided on theupper fixture unit 250 to be connected to a current detecting section280. The conductive probe 257 is rendered into contact with the terminal223 a connected to the ground layer 223 of the work 220 as the upperfixture unit 250 is moved to the work 220 positioned at a test position,thereby securing electric connection between the ground layer 223 andthe current detecting section 280. In this modification, since it is notrequired to provide electric connection between the lower fixture unit240 and the current detecting section 280, a conductive probe 281 whichis provided in the apparatus of the second embodiment as shown in FIG. 8is omitted. Since the arrangement of the first modification of thesecond embodiment is substantially identical to the arrangement of thesecond embodiment shown in FIG. 8 except the above mentionedconfiguration, the elements of the first modification which areidentical to those of the second embodiment are denoted with the samereference numerals, and a description thereof is omitted herein.

[0161] In this modification, the ground layer 223 is electricallyconnected to the current detecting section 280 via the conductive probe257. Each of the wirings 222 formed on the circuit board is capacitivelycoupled to the ground layer 223. In this way, the ground layer 223satisfies a requirement as the second electrode portion, namely, therequirement that the second electrode should be connected to an externalpower source and capacitively coupled to a target wiring inside thecircuit board. Thus, the ground layer 223 can be functioned as thesecond electrode portion in the first modification.

[0162] Operations of the testing apparatus as the first modification arethe same as those of the testing apparatus in accordance with the secondembodiment as shown in FIG. 8 except the following.

[0163] Specifically, in the first modification, a current due tophotoelectric effect runs through the current detecting section 280 fromthe ground layer 223 via the conductive probe 257, whereas in the secondembodiment, a current runs through the current detecting section 280from the metallic plate 241 via the conductive probe 281. The otheroperations of the testing apparatus in the first modification are thesame as the testing apparatus in accordance with the second embodimentshown in FIG. 8. The first modified testing apparatus enables toprecisely and stably test as to whether a target wiring is in a shortcircuit state or an open circuit state, and test as to whether there isa continuity between a selected pair of terminals.

[0164] In the first modification, it is judged whether each wiring is ina short circuit state or an open circuit state based on an amount ofelectric charges that have been charged in a capacitor comprised of theground layer 223 and each wiring. Thus, since the target wiring and thesecond electrode portion are provided on the same circuit board, thereis no likelihood that the capacity may vary due to a positionaldisplacement of the circuit board relative to the lower fixture unitwhen placing the circuit board to a test position, a warp or a variationof thickness over the entirety of the circuit board. As a result, thisarrangement enables to perform a wiring test precisely and stably.

[0165] In this modification, described is the case where the terminal223 a connected to the ground layer 223 is formed on the upper surfaceof the circuit board 220. This invention is applicable to a modificationother than the aforementioned modification. For instance, as far as thecircuit board 220 has a ground terminal on the lower surface thereof, itmay be possible to electrically connect a ground layer 223 to a powersource 270 or allow a conductive probe 257 to come into contact with theground terminal through the lower surface of the work 220 byconstructing a lower fixture unit with a metallic plate 241 which is notformed with an insulating film and rendering the ground terminal intocontact with the metallic plate 241.

[0166] In this modification, the ground layer formed inside the circuitboard 220 serves as the second electrode portion. Alternatively, aground plane which is so formed as to cover generally an entirety of onesurface of a circuit board, or a wiring other than the ground layerformed in the circuit board, e.g., a wiring serving as a power line maybe used as the second electrode portion.

[0167]FIG. 16 is a diagram showing a testing apparatus as a secondmodification of the second embodiment of the invention. The arrangementand operations of the testing apparatus in the second modification arebasically the same as those of the testing apparatus in accordance withthe second embodiment shown in FIG. 8. However, the manner of capturingthe photoelectron is different between the second modification and thesecond embodiment, and part of the arrangement of the secondmodification differs from the second embodiment in association with thedifference in the manner of photoelectron capturing. Accordingly, onlythe differences between the second modification and the secondembodiment are described herein. Elements of the second modificationwhich are identical to those of the second embodiment are denoted withthe same reference numerals, and a description thereof is omittedherein.

[0168] The testing apparatus as the second modification of the secondembodiment is adapted for testing an electric state of a circuit board230. As shown in FIG. 16, the circuit board 230 is constructed in such amanner that a plurality of wirings 232 are formed on a base plate 231.Each wiring 232 includes terminals 232 a and 232 b which are formed onthe respective opposite surfaces of the circuit board 230 to beconnected to an electronic component mounted on the circuit board or anexternal wiring, and a conductive portion 232 c which is formed on thesurface or inside the circuit board 230 to be connected to the terminals232 a, 232 b. In this modification, described is a case where thecircuit board 230 having the above construction is used as a work to betested by the testing apparatus. It is needless to say that the work isnot limited to the above circuit board.

[0169] In this modification, similar to the testing apparatus inaccordance with the second embodiment shown in FIG. 8, an upper fixtureunit 250 is moved toward the work 230 to securely hold the work 230between the upper fixture unit 250 and the lower fixture unit 240,whereby an airtight closed space SP is defined by a housing 251, a sealmember 252 and the work 230. The housing 251 is so constructed as toexpose a terminal 233 b-1 of a wiring 233 b including the terminal 233b-1 and a terminal 233 b-2, outside the closed space SP and accommodatethe terminal 233 b-2 inside the closed space SP. The upper fixture unit250 is provided with a conductive probe 258, and is connected to theplus terminal of a power source 270. The conductive probe 258 is adaptedto electrically connect the terminal 233 b-1 and the plus terminal ofthe power source 270 as the upper fixture unit 250 is moved to the work230 positioned at the test position. Thereby, a voltage of the powersource 270 is applied between the wiring 233 b connected to the terminal233 b-1 and a metallic plate 241 serving as the second electrode portionof this invention. When the voltage is applied, an electric field isgenerated in the vicinity of the terminal 233 b-2 which is connected tothe wiring 233 b and is accommodated in the closed space SP. Then, whena controller 201 selects a wiring 233 a as a target wiring, and anelectromagnetic wave irradiator 260 irradiates an electromagnetic wave Lonto a terminal potion 233 a-1 of the target wiring 233 a, electrons aredischarged from the terminal 233 a-1 and electrically attracted andcaptured on the terminal 233 b-2 aided by the existence of the electricfield. As a result, a current runs through the power source 270 via theconductive probe 258. At this time, electrons run through the metallicplate 241 which is capacitively coupled to the target wiring 233 a, fromthe power source 270 via the current detecting section 280 and theconductive probe 281. As a result, a current is detected by the currentdetecting section 280. Thus, a wiring test can be performed by thetesting apparatus of the second modification in the similar manner asthe testing apparatus in accordance with the second embodiment of theinvention.

[0170] As mentioned above, in the second modification of the secondembodiment, the upper fixture unit 250 is constructed in such a mannerthat the terminal 233 b-1 of the wiring 233 b formed on the circuitboard 230 is exposed outside the closed space SP, and the oppositeterminal 233 b-2 of the wiring 233 b is housed inside the closed spaceSP. Electrically connecting the terminal 233 b-1 to the power source 270via the conductive probe 258 in the above arrangement enables tofunction the wiring 233 b as the first electrode portion to capture thephotoelectron discharged from the terminal irradiated by electromagneticwave. As a result, this modification does not require a plate electrode253 which is provided in the testing apparatus in accordance with thesecond embodiment shown in FIG. 8, and the housing 251 is so configuredas to secure a minimal surface area for covering the terminal of awiring to be tested with respect to the work 230. Accordingly, thisarrangement enables a smaller testing apparatus while reducing a volumeof the closed space SP which is subjected to depressurization. Thus, awiring test can be performed in a shorter time because a time requiredfor depressurization is shortened due to the reduced volume of theclosed space SP.

[0171] This invention is not limited to the aforementioned embodimentsand modifications. Various modifications and alterations can beprovided. For instance, in the second embodiment and its modifications,described is the case where the interior of the housing isdepressurized. Alternatively, a depressurization may not be required orthe vacuum degree may be varied as the case may be. Further, in thesecond embodiment and its modifications, the housing is so configured asto cover the terminal of the target wiring formed on the surface of thecircuit board so as to irradiate an electromagnetic wave onto theterminal. Alternatively, there may be provided an arrangement in which aclosed space is defined by fitting contact of an outer circumferentialportion of a housing with an outer circumferential portion of a lowerfixture unit and the entirety of a circuit board is housed in the closedspace for depressurization. As a further altered form, a housing may beso configured as to house a circuit board and a lower fixture unit as awhole so as to depressurize the entire interior of the housing.

[0172] Further, combination of the modifications of the secondembodiment may be applicable. For instance, combining the firstmodification and the second modification enables to function a wiringformed on a circuit board (i.e., work) which is connected to a powersource as the first electrode portion and function a ground layer formedon the circuit board as the second electrode portion so as to perform awiring test.

[0173] As mentioned above, in the second embodiment and itsmodifications, since a high potential is applied to the first electrodeportion which is disposed in the vicinity of the terminal to beconnected to the target wiring, electrons discharged from the terminaldue to photoelectric effect upon irradiation of an electromagnetic waveare securely attracted and trapped on the first electrode portion.Furthermore, since the second electrode portion is so arranged as to becapacitively coupled to the target wiring, the electrons that have runthrough the first electrode portion are securely detected as a currentrunning through a closed circuit via the capacitor comprises of thetarget wiring and the second electrode portion. Thus, a wiring test isperformed based on the detected current. This arrangement enablestesting of opens and shorts-circuit of the target wiring withoutelectric contact of both surfaces of the circuit board with the upperand lower fixtures.

[0174]FIG. 17 is a diagram showing a circuit board testing apparatus inaccordance with a third embodiment of the invention. FIG. 18 is a blockdiagram showing an electric configuration of the testing apparatus inFIG. 17. A circuit board testing apparatus is adapted to test a circuitboard 410 which is capable of mounting thereon a semiconductor chipaccording to C4 (Controlled Collapse Chip Connection) package method.

[0175] As shown in FIG. 17, the circuit board 410 is constructed in sucha manner that a plurality of wirings as represented by the wiring 412are formed on a base plate 411. Each wiring 412 includes a pad portion412 a formed on one surface of the base plate 411 to be connected with apad on a semiconductor chip, a ball grid portion 412 b formed on theopposite surface of the base plate 411, and a conductive portion 412 carranged on or in the base plate 411 to electrically connect the padportion 412 a and the ball grid portion 412 b. The pad portions 412 aare arranged at small pitches to correspond to the pitches of the padsof semiconductor chips, whereas the ball grid portions 412 b arearranged at larger pitches as compared to the pitches of the padportions 412 a. The pad portions 412 a are gathered in a region ER onone surface of the circuit board 410. The region ER is a wiring endexposure area. In this embodiment, the circuit board 410 having theabove construction is referred to as a work to be tested by theapparatus. However, it is needless to say that a circuit board to betested by the present embodiment is not limited to the above.

[0176] The apparatus includes a work holder 421 to carry a piece ofcircuit board as a work 410. The work holder 421 is movable between atest position (position shown in FIG. 17) where the work 410 is testedand a load/unload position (not shown) where the work 410 is loadable tothe work holder 421 or unloadable from the work holder 421. A workdriving mechanism 422 drivingly reciprocate the work holder 421 back andforth between the test position and the load/unload position in responseto a control signal from a controller 430 which controls an overalloperation of the apparatus.

[0177] A lower fixture unit 440 is provided below the work 410 at thetest position. The lower fixture unit 440 includes a plurality ofconductive spring probes 441 which are arranged to respectivelyconnected with the corresponding ball grid portions 412 b of therespective wrings 412. The lower fixture is further provided with amultiplexer 442, and a lower fixture base (not shown) which is movabletoward and away from the work 410 while holding the probes 441 and themultiplexer 442 thereon. The lower fixture base is coupled to a lowerfixture unit driving mechanism 445. The lower fixture unit drivingmechanism 445 drivingly moves the lower fixture base toward and awayfrom the work 410 in accordance with a control signal from thecontroller 430.

[0178] An upper fixture unit 450 is arranged above the work 410 at thetest position. The upper fixture unit 450 includes a cap-like housing ofa transparent glass which is formed with an exhaust port 454, and is soconfigured as to cover the wiring exposure area ER on the work 410. Theupper fixture 450 further includes a seal member 452 mounted on an endportion of a side wall of the housing 451, and a transparent electrode453 mounted on an inner upper surface of the housing 451. Thetransparent electrode 453 extends in two dimensions to substantiallycover the wiring exposure area ER. These elements 451 through 454 areintegrally movable toward and away from the work 410. An upper fixtureunit driving mechanism 456 is coupled to the upper fixture unit 450. Theupper fixture unit 450 is moved toward and away from the work 410 inresponse to a control signal from the controller 430.

[0179] The upper fixture unit 450 is moved to the work 410 until theseal member 452 of the housing 451 comes into contact with the surfaceof the work 410. As a result, the seal member 452 is resilientlydeformed while being pressingly sandwiched between the bottom edge ofthe side wall of the housing 451 and the surface of the work 410.Consequently, an airtight closed space SP is defined by the work 410,the seal member 452, and the housing 451.

[0180] The exhaust port 454 formed in the housing 451 is communicatedwith an exhausting device 490 via an exhaust pipe (not shown). When theexhausting device 490 is activated based on a control signal from thecontroller 430, the air inside the closed space SP is exhausted tothereby render the interior of the closed space SP to a depressurizedstate. When a test is performed, the closed space SP is preferably heldat a vacuum degree of about 10⁻² atm as is the same as in the abovedescribed embodiments.

[0181] A power source 460 is provided in the apparatus to apply acertain DC voltage to a target wiring. The plus terminal of the powersource 460 is electrically connected to the transparent electrode 453,and the minus terminal thereof is connected to the multiplexer 442 via acurrent detecting section 480. The multiplexer 442 is operated to selecta ball grid portion of a wiring in response to a selection command fromthe controller 430. In this configuration, when, for example, as shownin FIG. 17, a ball grid portion 521 b of a wiring 521 is selected basedon a selection command from the controller 430, a DC voltage of thepower source 460 is applied between the ball grid portion 521 b and thetransparent electrode 453. In this case, the wiring 521 is a targetwiring to be tested. A current value measured by the current detectingsection 480 is converted into a digital signal by an A/D convertercircuit 481, and sent to the controller 430. Thereupon, the controller430 determines whether the target wiring is in continuity or not basedon the measured current value while controlling an overall operation ofthe apparatus.

[0182] A UV lamp 470 is provided above the upper fixture unit 450. Alamp control circuit 471 controls the UV lamp 470 to turn on and offbased on a control signal from the controller 430. The UV lamp 470 emitsan ultraviolet laser light beam L toward the upper surface of thehousing 451. An ultraviolet laser light beam L emitted from the UV lamp470 passes through the upper surface of the housing 451 and thetransparent electrode 453, and is incident upon the wiring exposure areaER on the work 410.

[0183] In this embodiment, the UV lamp 470 is used as an electromagneticwave irradiator. Alternatively, as far as an element is capable ofcausing a conductive member of a wiring on a circuit board to exhibit aphotoelectric effect, such an element is usable as an electromagneticwave irradiator. The UV lamp 470 is operable to emit ultraviolet laserlight beams having a wavelength of 266 nm.

[0184] In this embodiment, ultraviolet laser light beams are emittedusing the UV lamp 470 for the purpose of raising photoelectric effect.However, this invention is not limited to a UV lamp, and visible lightbeams, infrared light beams or its equivalent may be used.

[0185] Next, an open circuit test with respect to a wiring by thetesting apparatus in accordance with the third embodiment is describedwith reference to FIGS. 20 and 21. FIG. 20 is a flowchart showingoperations of the circuit board testing apparatus shown in FIG. 17. Whenan untested work (circuit board) 410 is loaded on the work holder 421 atthe load/unload position by a handling device (not shown) incorporatedin the testing apparatus or a manual operation by an operator (in StepU1), the controller 430 starts to control operations of the variousparts of the apparatus to execute the following steps U2 to U12 so as totest open-circuit of the wirings on the work 410.

[0186] First, the work holder 421 clamps the work 410 thereon in StepU2. Then, while holding the work 410 thereon, the work holder 421 ismoved to the test position (position shown in FIG. 17) where the work410 is tested (in Step U3). Thus, the work 410 is positioned at the testposition.

[0187] Subsequently, the upper fixture unit 450 and the lower fixtureunit 440 are moved to the work 410 (in Step U4). As the lower fixtureunit 440 is moved to the work 410, as shown in FIG. 17, lead ends ofconductive spring probes 441 are pressed against respectivecorresponding ones of the ball grid portions 412 b of the wirings 412 tobe electrically connected thereto. Simultaneously, the upper fixtureunit 450 is moved to the test position, as shown in FIG. 17 to securelyholds the work 410 between the upper fixture unit 450 and the lowerfixture unit 440. Next, an exhausting device 490 is activated todepressurize the interior of the closed space SP defined by the housing451, the seal member 452 and the work 410 (in Step U5).

[0188] Thus, when the apparatus is set up for testing the work 410, theUV lamp 470 is turned on to irradiate an ultraviolet laser light beam Lonto the wiring exposure area ER (in Step U6). Then, the apparatusimplements an open circuit test with respect to the target wiring (inStep U7) to test the work 410. The open circuit test are described laterin detail.

[0189] Upon completion of the open circuit test, the UV lamp 470 isturned off (in Step U8). Then, the activation of the exhausting device490 is suspended, and the air outside the apparatus is drawn into theclosed space SP (in Step U9). Subsequently, the lower fixture unit 440and the upper fixture unit 450 are moved away from the work 410 (in StepU10), and the work holder 421 releases clamping the work 410 and isretracted to the load/unload position (in Step U11). Lastly, when it isverified that the work 410 after the open circuit test is unloaded fromthe work holder 421 (in Step U12), the routine returns to Step U1 toimplement the aforementioned series of operations.

[0190] Next, the open circuit test with respect to a wiring to beimplemented by the apparatus in accordance with the third embodiment(Step U7) is described in detail with reference to FIG. 20. FIG. 20 is aflowchart showing an open circuit test with respect to a wiring to beimplemented by the circuit board testing apparatus in accordance withthe third embodiment. After the UV lamp 470 is turned on in Step U6, themultiplexer 442 selects an arbitrary wiring 521 as a target wiring inaccordance with a selection command from the controller 430 toelectrically connect the target wiring 521 to the power source 460, anda voltage is applied between the ball grid portion 521 b of the targetwiring 521 and the transparent electrode 453 (in Step U71). Upon lapseof a time until the power supply is stabilized (in Step U72), thecurrent detecting section 480 measures a current running therethrough(in Step U73). When a voltage is applied between the ball grid portion521 b and the transparent electrode 453 in a state that the targetwiring 521 is in continuity, an electric field is generated between thetransparent electrode 453 and the pad portion 521 a. At this time,electrons discharged from the pad portion 521 a due to photoelectriceffect are electrically attracted and captured by the transparentelectrode 453 aided by the existence of the electric field. As a result,a photocurrent I_(O) runs through a conductive pathway which isestablished from the plus terminal of the power source 460 to the minusterminal thereof via the transparent electrode 453, the target wiring521, the multiplexer 442, and the current detecting section 480, and isdetected by the current detecting section 480. On the other hand, in thecase where the target wiring 521 is in an open circuit state, theaforementioned conductive pathway is not established, and the currentvalue detected by the current detecting section 480 is zero orexceedingly lower than a current detected in the case where the targetwiring 521 is in continuity.

[0191] In this way, the controller 430 determines, as mentioned below,whether the target wiring is in an open circuit state or not based onthe current value detected by the current detecting section 480 (in StepU74). Specifically, in the case where the photoelectric current I_(O)detected by the current detecting section 480 is equal to or greaterthan a predetermined threshold value I1, it is judged that the targetwiring is continuous. On the other hand, if the photocurrent I_(O) islower than the threshold value I1, it is judged that the target wiringis discontinuous. In this way, in the third embodiment, the controller430 has a function of a determinator as well as other function ofcontrolling the operation of the apparatus. The threshold value I1 isdetermined as follows. Since the magnitude of photoelectric current isdetermined by multiplying intensity of irradiated electromagnetic wavei.e. light by the surface area of a conductive member irradiated withthe light, the threshold value I1 is selected from a range smaller thana minimal current value which is theoretically calculated based onintensity of an ultraviolet laser light beam L and the surface area ofthe pad portion 412 a and larger than a noise current value in order tosecurely distinguish the photoelectric current from the other noisecurrents.

[0192] In this way, when an open circuit test with respect to one wiringis completed, the routine returns to Step U71 to implement the opencircuit test of another wiring. Thus, the aforementioned series ofoperations are repeated until the test is completed with respect to allthe wirings of the circuit board.

[0193] As mentioned above, the apparatus shown in FIG. 17 is similar tothe prior art arrangement in the aspect of testing whether a wiring isin an open circuit state by utilizing photoelectric effect. However, theapparatus of the third embodiment has the feature that a plurality ofpad portions 412 a formed on the upper plane of the work 410 areirradiated with ultraviolet laser light beams. The apparatus isadvantageous in that an open circuit test can be performed with asimplified apparatus and within a short time without requiring anarrangement of focusing or scanning ultraviolet laser light beams.

[0194] Generally, a wiring formed on a circuit board defines a capacitorhaving a floating capacity between the wiring and a GND pad or betweenthe wiring and the other wiring. Consequently, when a voltage isapplied, a transient current runs through the wirings in an attempt tocharge the capacitor. As a result, it is highly likely that an erroneousjudgment is made resulting from erroneous detection of a transientcurrent by the current detecting section 480. In view of this, thisembodiment employs an arrangement in which a current is measured afterimplementing Step U72, namely, upon lapse of a certain stand-by timefrom application of a voltage until a current is stabilized. Theadditionally provided stand-by time, however, may extend a time requiredfor a test. In view of this, the following first modification of thethird embodiment is devised in order to shorten the test time.

[0195]FIG. 21 is a diagram showing a testing apparatus as the firstmodification of the third embodiment to suppress a transient current soas to shorten the stand-by time. The first modification is differentfrom the third embodiment in that in the first modification, respectiveswitch portions of a multiplexer 442 include normal close (NC) contactsand that the wirings other than a target wiring are connected to theminus terminal of a power source 460 bypassing a current detectingsection 480 through the NC contacts. The first modification is similarto the third embodiment in that a wiring 521 selected as a target wiringis connected to the current detecting section 480 through a normal open(NO) contact. Since an arrangement of the first modification isidentical to that of the third embodiment except the above points,elements of the first modification which are identical to those of thethird embodiment are denoted at the same reference numerals, and adescription thereof is omitted herein.

[0196] Operations of the first modification are substantially the sameas those of the testing apparatus shown in FIG. 17 (as shown in theflowcharts of FIGS. 19 and 20) except the following points.Specifically, in the first modification, when a voltage is appliedbetween a ball grid portion 521 b of a target wiring 521 and atransparent electrode 453, an electric field is generated between padportions 522 a, 523 a of the other wirings to which a GND potential orground potential is applied, and the transparent electrode 453. As aresult, electrons discharged from the pad portions 521 a, 522 a and 523a due to photoelectric effect are electrically attracted and captured bythe transparent electrode 453, whereby a current runs through thewirings. The current running through the target wiring 521 is guided tothe current detecting section 480 via the NO contact of the switchportion 443 a of the multiplexer 442. On the other hand, the currentsrunning through the other wirings 522, 523 are guided to the minusterminal of the power source 460 via the respective NC contacts of theswitch portions 443 b and 443 c of the multiplexer 442. This arrangementenables to eliminate a drawback that a transient current which hasundesirably run through the other wirings 522 and 523 may run throughthe current detecting section 480, and eliminates a likelihood that thetransient current may adversely affect current detection by the currentdetecting section 480.

[0197] As mentioned above, the testing apparatus shown in FIG. 21 is soconstructed as to keep a current running through the wirings other thanthe target wiring from running through the current detecting section480. This arrangement eliminates an erroneous judgment resulting fromrunning of a transient current through the current detecting section 480even if the stand-by time is shortened, and consequently shortens a timerequired for a test as a whole.

[0198] The manner of suppressing running of a transient currentdescribed in the above first modification can be modified as shown inthe following second and third modifications.

[0199] In the above, description is made about the open circuit test ofa wiring, implemented by the circuit board testing apparatus accordingto the third embodiment. The apparatus according to the third embodimentcan perform a short circuit test of the wirings by supplying testsignals through the ball grid portions. For example, if the plusterminal of the power source 460 is connected to the ball grid connectedto the wiring 523 and the minus terminal of the power source isconnected to the ball grid connected to the wiring 522, then, the shortcircuit between the wirings 523 and 522 is detected. According to thesecond and third modifications of the third embodiment, an open circuittest with respect to a target wiring, and a short circuit test withrespect to the target wiring relative to the other wiring can beperformed simultaneously.

[0200]FIG. 22 is a diagram showing a circuit board testing apparatus asa second modification of the third embodiment, and FIG. 23 is aflowchart showing operations of an open/short circuit test by theapparatus shown in FIG. 22. The second modification is different fromthe third embodiment in that in the second modification, respectiveswitch portions 443 of a multiplexer 442 have normal close (NC)contacts, and that wirings other than a target wiring are connected tothe plus terminal of a power source 460 via the NC contacts. The secondmodification is similar to the third embodiment in that the wiringselected as the target wiring is connected to a current detectingsection 480 via a normal open (NO) contact. Since the arrangement of thesecond modification is identical to that of the third embodiment exceptthe above points, elements of the second modification which areidentical to those of the third embodiment are denoted with the samereference numerals, and a description thereof is omitted herein.

[0201] Operations of the second modification are substantially the sameas those of the testing apparatus shown in FIG. 17 (as shown in theflowchart of FIG. 19) except that the open/short circuit test operationshown in FIG. 23 is executed in the second modification in place of theopen circuit test implemented in Step U7 of FIG. 19. The operations ofthe second modification are described with reference to FIGS. 19, 22,and 23.

[0202] When a UV lamp 470 is turned on in Step U6 of FIG. 19, eachwiring is connected to the plus terminal of the power source 460 viaeach NC contact of each switch portion 443 of the multiplexer 442 toapply the same potential thereto as the transparent electrode 453. Next,the multiplexer 442 selects one wiring 521 in response to a selectioncommand from a controller 430 in Step U711 (namely, the switch portion443 a is switched over to the NO contact) to connect the wiring 521 tothe current detecting section 480. As a result, merely the wiring 521 isset to a low potential. Upon lapse of a time until a fluctuation ofcurrent detection due to a transient current becomes negligible (in StepU712), the current detecting section 480 measures a current runningtherethrough (in Step U713).

[0203] Here, described is a case where the wiring 521 is short-circuitedwith one of the other wirings 521 and 523. For instance, in the casewhere the wiring 521 is short-circuited with the wiring 522 at a portiony shown by the dotted line in FIG. 22, a conductive pathway isestablished through which a current runs from the power source 460 andis returned thereto via the wiring 522, the short-circuited portion y,the target wiring 521 and the current detecting section 480. As aresult, a short-circuit current I_(S) runs through the conductivepathway, and the current value is measured by the current detectingsection 480.

[0204] On the other hand, in the case where the wiring 521 is notshort-circuited with the other wiring, a current value measured by thecurrent detecting section 480 is determined based on presence or absenceof an open circuit portion in the wiring 521 as in the case of theapparatus shown in FIG. 17. If the wiring 521 is in a normal continuousstate (namely, there is no open circuit portion in the wiring 521 and noshort circuit portion in the wiring 521 relative to the other wiring), aphotoelectric current I_(O) runs through the current detecting section480. On the other hand, if there is a short-circuited portion betweenthe wiring 521 and at least one of the other wirings, a short circuitcurrent I_(S) runs through the current detecting section 480. Further,if there is an open circuit portion in the wiring 521, the currentmeasured by the current detecting section 480 is zero or exceedinglylower than the current due to the photoelectric current I_(O).

[0205] As mentioned above, generally, a short circuit current I_(S) isdistinguishably larger than the photoelectric current I_(O).Accordingly, the controller 430 determines whether the target wiring isin an open circuit state or a short circuit state based on the currentin Step U714. Specifically, if the current value detected by the currentdetecting section 480 is lower than a threshold value I1, it is judgedthat the wiring 521 is in an open circuit state. If the current valuedetected by the current detecting section 480 is not smaller than thethreshold value I1 and smaller than a threshold value I2, it is judgedthat the wiring 521 is in a normal continuous state. On the other hand,if the current value detected by the current detecting section 480 isnot smaller than the threshold value I2, it is judged that the wiring521 is short-circuited with at least one of the other wirings. Thethreshold value I1 is determined in the similar manner as in the thirdembodiment.

[0206] The threshold value I2 is selected from a range larger than apossible maximal value of the photoelectric current and smaller than apossible minimal value of the short-circuit current in order todistinguish the photoelectric current from the short circuit currentwithout fail. The maximal value of the photoelectric current may beestimated theoretically from the multiplication of an intensity ofultraviolet laser light beam L by a surface area of the pad portion 412a irradiated by the light. The minimal value of the short circuitcurrent is theoretically estimated from the multiplication of adimension of a short-circuited portion of the wirings by an appliedvoltage, with the dimension of the short-circuited portion beinginferred from the design and production of the circuit board under test.

[0207] In this way, when an open/short circuit test of a wiring iscompleted, the routine returns to Step U711, and the aforementionedseries of operations are implemented with respect to another wiring.Thus, the aforementioned series of operations are repeated until thetest is completed with respect to all the wirings on the circuit board.The other operations implemented by the apparatus in the secondmodification are the same as those implemented by the apparatus shown inFIG. 17.

[0208] As mentioned above, the apparatus shown in FIG. 22 is operated tojudge whether the target wiring is in continuity based on a differencebetween a photoelectric current I_(O) running in the case where thetarget wiring is in a normal continuous state and a short circuitcurrent I_(S) running in the case where the target wire is in a shortcircuit state with the other wiring. This arrangement enables an opencircuit test of the target wiring and a short circuit test of the targetwiring relative to the other wiring simultaneously.

[0209] In the apparatus shown in FIG. 22, in the case where the targetwiring has an open circuit portion x and a short circuit portion y atthe same time, the current value detected by the current detectingsection 480 is about the level of a short circuit current I_(S).Therefore, the controller 430 may prioritize the judgment that there isa short circuit portion in the target wiring, and resultantly misjudgethat there is no open circuit portion. Further, if the short circuitcurrent I_(S) is about the same level as that of the photoelectriccurrent I_(O) due to a large electric resistance at the short circuitportion, the controller 430 may misjudge that the target wiring is in anormal continuous state despite the fact that there is a short circuitportion.

[0210] In view of the above, a third modification of the thirdembodiment is proposed to solve the aforementioned drawback. FIG. 24 isa diagram showing an apparatus as the third modification, and FIG. 25 isa flowchart showing operations of an open/short circuit test to beimplemented by the apparatus shown in FIG. 24. The apparatus of thethird modification enables detection of both open circuit and shortcircuit by implementing short circuit test after the test of continuityof a target in the manner as is done by the second modification of thethird embodiment. The arrangement of the third modification is identicalto that of the second modification except the following points. In thethird modification, a changeover switch 444 is additionally provided toswitch over the NC contacts of switch portions 443 of a multiplexer 442between the plus terminal and the minus terminal of a power source 460to render each switch portion 443 to be selectively connected to bothterminals or poles of the power source 460. Since the other arrangementof the third modification is identical to that of the secondmodification, elements of the third modification which are identical tothose of the second modification are denoted with the same referencenumerals, and a description thereof is omitted herein.

[0211] An open/short circuit test to be implemented by the apparatus ofthe third modification is described with reference to FIGS. 24 and 25.First, at an initial stage of the test, the changeover switch 444 is setto a contact a with all the switches of multiplexer 442 being set to NCterminals to connect all the wirings on a circuit board 410 to the minusterminal of the power source 460 bypassing the a current detectingsection 480. Then, in Step U721, the multiplexer 442 is operated toselect one wiring 521 in response to a selection command from acontroller 430 to connect the wiring 521 to a current detecting section480. Upon lapse of a time until a fluctuation of current detection dueto a transient current becomes negligible (in Step U722), the currentdetecting section 480 measures a current running therethrough (in StepU723). Then, the controller 430 judges whether the wiring 521 is in anopen circuit state or not based on the measured current value.

[0212] Subsequently, the changeover switch 444 is switched over to acontact b, and a plus potential is applied to wirings 522 and 523 whichare the wirings other than the target wiring 521 selected for thetesting (in Step U725). Thereafter, upon lapse of a time (in Step U726),the current detecting section 480 measures a current runningtherethrough in substantially the same manner as that of the open/shortcircuit test implemented by the apparatus shown in FIG. 22, (in StepU727). Similar to the apparatus shown in FIG. 22, the controller 430judges whether there is a short circuited portion between the wiring 521and the other wirings, based on the measured current value. Thus, uponcompletion of the open circuit test with respect to the wiring 521 andthe short circuit test between the wiring 521 and the other wirings, thechangeover switch 444 is switched over to the contact a again (in StepU729). The aforementioned series of operations are repeated until theopen/short circuit test is completed with respect to all the wirings onthe circuit board 410 (in Step U739).

[0213] As mentioned above, the apparatus shown in FIG. 24 is arranged toperform an open circuit test and then a short circuit test by utilizingphotoelectric effect. This arrangement enables to perform an opencircuit test with respect to a target wiring and a short circuit testbetween the target wiring and the other wirings without the drawbacksthat an open circuit portion is neglected due to the presence of ashort-circuited portion and that an erroneous judgment that the targetwiring is in a normal continuous state is made despite the fact thatthere is a short-circuited portion in the target wiring.

[0214] In the third embodiments, the ultraviolet laser light is notnecessary in the short circuit test. In view of this, the arrangement ofthe third modification may be so configured as to turn off the UV lamp470 after the open circuit test. However, it is preferable to stabilizethe intensity of ultraviolet laser light beam in order to perform aprecise test. To this end, it is practically desirable to keep turningthe UV lamp 470 on until the open circuit test is completed with respectto all the wirings formed on at least one work 410.

[0215] In the third modification, the changeover switch 444 is providedto selectively connect both terminals of the power source to the wiringsother than the target wiring. Another arrangement is available toselectively connect both terminals of the power source to the wiringsother than the target wiring. For instance, it may be possible toprovide an additional contact for each switch portion 443 of themultiplexer 442 in the apparatus shown in FIG. 22, connecting theadditional contact to the minus terminal of the power source 460.Switching over of the switch portions 443 enables selective switch overof the voltages applied to the other wirings while selecting the targetwiring.

[0216]FIG. 26 is a diagram showing a fourth modification of the thirdembodiment. The basic principle of the testing according to the fourthmodification is the same as the third embodiment except the manner ofapplying a voltage from a power source and the manner of collecting orcapturing the electrons discharged by the photoelectric effect.Accordingly, elements of the fourth modification that are identical tothose of the third embodiment are denoted at the same referencenumerals, and the fourth modification is described primarily focusing ondifferences between the fourth modification and the third embodiment.

[0217] The testing apparatus as the fourth modification is not providedwith an electrode on the housing 451 for trapping or capturingphotoelectrons and is so configured as to trap or capture electronsdischarged from a target wiring, by applying a voltage to all or part ofwirings formed around the target wiring. To this end, in the fourthmodification, the plus terminal of a power source 460 is connected toeach NC contact of each switch portion 443 of a multiplexer 442, and theminus terminal thereof is connected to each NO contact of each switchportion 443 of the multiplexer 442 via a current detecting section 480.

[0218] Here, described is a case where, as shown in FIG. 26, a switchportion 443 a connected to a wiring 521 of the multiplexer 442 isconnected to the NO contact to make the wiring 521 a target wiring to betested. In this case, if the wiring 521 is in a normal continuous state,an electric field is generated between pad portions 412 a of the wiringsother than the target wiring 521, and a pad portion 521 a of the targetwiring 521 when a voltage is applied between the target wiring 521 andthe other wirings. Electrons discharged from the pad portion 521 a ofthe target wiring 521 due to photoelectric effect by irradiation ofultraviolet laser light beam are electrically attracted by the padportion 512 a aided by the existence of the electric field or potential.In the above state, if the target wiring 521 is in continuity, aconductive circuit pathway is established through which a current runsfrom the power source 460 and returns thereto via the other wirings andthe target wiring 521. Thus, a current running through the target wiring521 is measured by the current detecting section 480.

[0219] On the other hand, if the target wiring 521 is not in continuityi.e. open circuited, the aforementioned conductive pathway is notestablished, and a current value detected by the current detectingsection 480 is zero or exceedingly lower than a current value detectedin the case where the wiring 521 is in continuity.

[0220] As mentioned above, the testing apparatus shown in FIG. 26performs an open circuit test of wirings in the similar manner as thatof the apparatus shown in FIG. 17. In the fourth modification, it is notrequired to provide an electrode inside a housing 451. Therefore, thehousing 451 may be configured to have such a dimension as to cover awiring terminal or pad espousing area ER on a work 410 and to enclose aminimal space above the area ER. This arrangement enables a compact sizeof the apparatus while simplifying the construction of the apparatus.Further, since the volume of a closed space SP defined by the housing451, seal member 452, and the work 410 is reduced, a time required fordepressurizing the interior of the closed space SP is shortened with theresult that a test by the apparatus can be implemented in a short time.

[0221] In the fourth modification, it is required to perform a shortcircuit test between ball grid portions prior to an open circuit test.This is because if there is a short-circuited portion between the ballgrid portions, a short circuit current may run through the currentdetecting section 480 and the current detecting section 480 may make amisjudgment that there is no open circuit portion in the target wiring.

[0222] In the fourth modification, it is preferable to use a pluralityof wirings formed around a target wiring as the wirings serving as anelectrode. This is because if a single wiring is used as the electrode,and the wiring has an open circuit portion, accurate test cannot beperformed with such a testing apparatus.

[0223] In the fourth modification, it is possible to reverse thepolarities of the power source 460 and to perform an open circuit testwith respect to a target wiring by setting the target wiring at a highpotential and setting the other wirings serving as an electrode at a lowpotential. Specifically, in this altered case, an electric field havinga direction of electron flow which is opposite to that of the fourthmodification is generated between the pad portions 421 a of the otherwirings and the pad portion 521 a of the target wiring 521. In thisaltered case, if there is an open circuit portion in the target wiring521, the aforementioned conductive pathway is not established, andaccordingly, the open circuit test with respect to the target wiring 521can be performed in the similar manner as the fourth modification.

[0224] This invention is not limited to the aforementioned embodimentsand modifications. Various modifications and alterations can beprovided. For instance, in the third embodiment and its modifications,the circuit board 410 to be tested as a work is of the type on which asemiconductor chip is mounted by C4 package method. Alternatively, thisinvention is applicable to test a circuit board in which one surface ofa base plate is formed with wirings or a circuit board formed with acuffed wiring pattern.

[0225] In the third embodiment and its modifications, as described isthe case where the interior of the housing is depressurized.Alternatively, a depressurization may not be required or the vacuumdegree may be varied according to needs. This application is based onpatent application Nos. 2001-42356, 2001-111132, and 2001-111133 filedin Japan, the contents of which are hereby incorporated by reference.

[0226] Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such change andmodifications depart from the scope or spirit of the invention, theyshould be construed as being included therein.

What is claimed is:
 1. A circuit board testing apparatus for testing aplurality of wirings formed on a circuit board, each wiring having afirst terminal and second terminal, the apparatus comprising: anelectromagnetic wave irradiator which irradiates an electromagnetic waveonto a first terminal of a selected one of the wirings to dischargeelectrons from the irradiated terminal by photoelectric effect; anelectrode disposed at such a position as to trap the dischargedelectrons; a voltage supplier which produces a difference of electricpotential between the electrode portion and the other terminal of theselected wiring so that the electrode portion has an electricalpotential higher than the other terminal of the selected wiring; acurrent detector which detects a current caused by electrons trapped bythe electrode and flows through the selected wiring via the electrode;and a judger which determines existence of open-circuit and/orshort-circuit based on the current detected by the current detector. 2.The circuit board testing apparatus according to claim 1, wherein theelectromagnetic wave irradiator includes a deflector which changes thedirection of the electromagnetic wave in such a manner as to selectivelyand successively irradiate the first terminals of the plurality ofwirings with the electromagnetic wave.
 3. The circuit board testingapparatus according to claim 1, wherein the voltage supplier includes apower source, and a connector which connects the power source, theelectrode, the second terminal of the selected wiring, and the currentdetector with one another to constitute a closed circuit thereby.
 4. Thecircuit board testing apparatus according to claim 3, wherein theconnector includes a plurality of probes which are to be brought intocontact with the respective second terminals of the plurality of wiringsto establish electrical connections therewith, and a switch whichselectively connects the plurality of probes to the power source.
 5. Thecircuit board testing apparatus according to claim 1, further comprisinga housing which encloses the first terminals of the plurality of wiringsto constitute an airtight closed space, and a depressurizer whichdepressurizes the closed space.
 6. The circuit board testing apparatusaccording to claim 5, wherein the upper portion of the housing istransparent, the electric magnetic wave irradiator is located above thehousing to irradiate the first terminals through the upper portion ofthe housing, and the electrode is formed on the housing.
 7. The circuitboard testing apparatus according to claim 6, wherein the electrodeincludes a transparent electrode formed on an upper portion of thehousing.
 8. The circuit board testing apparatus according to claim 6,wherein the electrode portion includes a meshed electrode formed on theupper portion of the housing, and the electromagnetic wave irradiatorirradiates the first terminal through the upper portion of the housingnot covered by the meshed electrode.
 9. The circuit board testingapparatus according to claim 6, wherein the side wall of the housing ismade of electrically conductive material to function as the electrode.10. The circuit board testing apparatus according to claim 1, whereinthe voltage supplier includes a power source having two poles, and aconnector which connects the selected wiring to one pole of the powersource, and at least a part of the unselected wirings to the other poleof the power source so that the wirings connected with the other pole ofthe power source serves as the electrode.
 11. The circuit board testingapparatus according to claim 1, wherein the voltage supplier includes apower source having at least two poles, and a switch arrangement whichconnects the selected wiring to one pole of the power source, and allthe unselected wirings to the other pole of the power source so that thewirings connected with the other pole of the power source serves as theelectrode.
 12. The circuit board testing apparatus according to claim 1,wherein the voltage supplier includes a power source, and a switch whichconnects the second terminal of the selected wiring to the power source.13. The circuit board testing apparatus according to claim 1, whereinthe voltage supplier includes a power source, and a switch whichconnects the power source to the second terminal of a wiring adjacent tothe selected wiring of which first terminal is irradiated by theelectromagnetic wave.
 14. A circuit board testing apparatus for testingcontinuity and/or short-circuit of wirings formed on a circuit board,each wiring having first and second terminals, the apparatus comprising:an electromagnetic wave irradiator which irradiates the first terminalsof the wirings with an electromagnetic wave to allow electrons to bedischarged from the first terminals by photoelectric effect; anelectrode arranged to trap discharged electrons; a voltage supplier forapplying a voltage between the electrode and the second terminals of thewiring in a manner that voltage at the electrode becomes higher thanvoltage at the second terminals of the wirings; a current detector whichdetects an electric current which is caused by electrons trapped by theelectrode and flows through the wirings via the electrode; and a judgerwhich determines existence of open-circuit and/or short-circuit of thewirings based on the current detected by the current detector.
 15. Thecircuit board testing apparatus according to claim 14, wherein theelectromagnetic wave irradiator is arranged to alternatively irradiatethe terminals one at a time, and the voltage supplier is to bealternatively connected with the second terminals of the wirings one ata time.
 16. A circuit board testing apparatus for testing a plurality ofwirings of a circuit board, each wiring including a terminals formed ona surface of the circuit board and an electric conductor formed on thesurface of or inside the circuit board and electrically connected to theterminal, the apparatus comprising: an electromagnetic wave irradiatorwhich selectively and successively irradiates terminals of the wiringswith an electromagnetic wave one by one to discharge electrons from theirradiated terminal by photoelectric effect; a first electrode disposedto trap the discharged electrons; a second electrode capacitivelycoupled to conductors of the plurality of wirings; a power sourceprovided between the first electrode and the second electrode to cause adifference of an electric potential in such a manner that the firstelectrode has a potential higher than that of the second electrode; acurrent detector which detects a current caused by the dischargedelectrons which flows through a circuit including the first electrode,the power source, and the capacitive coupling; and a judger which judgeswhether the electric conductor of the selected wiring is continuous,based on a current value detected by the current detector when the firstterminal is irradiated by the electromagnetic wave and another currentvalue detected by the current detector when another terminal isirradiated by the electromagnetic wave.
 17. A circuit board testingapparatus for testing a plurality of wirings formed on a circuit board,at least one of the wirings including a first and second terminalsformed on a surface of the circuit board and a conductive portion formedon the surface of the circuit board or inside the circuit board andelectrically connected to the terminals, the apparatus comprising: anelectromagnetic wave irradiator which selectively and successivelyirradiates terminals of the wirings with an electromagnetic wave one byone to discharge electrons from the irradiated terminal by photoelectriceffect; a first electrode portion disposed at such a position as to trapthe discharged electrons; a second electrode portion capacitivelycoupled to the plurality of wirings; a power source provided between thefirst electrode portion and the second electrode portion to cause adifference of electric potential in such a manner that the firstelectrode portion has a potential higher than that of the secondelectrode portion; a current detector which detects a current caused bydischarged electrons which flows through a circuit including the firstelectrode, the power source, and the capacitive coupling; and a judgerwhich judges whether a conductive portion of the selected wiring betweenthe first terminal and second terminal is continuous based on a currentvalue detected by the current detector when the electromagnetic wave isirradiated onto the first terminal of the target wiring and anothercurrent value detected by the current detector when the electromagneticwave is irradiated onto the second terminal of the target wiring.
 18. Acircuit board testing apparatus for testing a circuit board formed witha plurality of wirings, each wiring including a terminal formed on asurface of the circuit board and a conductive portion formed on thesurface of the circuit board or inside the circuit board andelectrically connected to the terminal, the apparatus comprising: anelectromagnetic wave irradiator which selectively and successivelyirradiates an electromagnetic wave onto terminals of the wirings one byone to discharge electrons from the terminal by photoelectric effect; afirst electrode disposed at such a position as to trap the dischargedelectrons; a second electrode capacitively coupled to at least a part ofthe selected wiring; a power source provided between the first electrodeand the second electrode to apply a voltage in such a manner that thefirst electrode has a potential higher than that of the secondelectrode; a current detector which detects a current caused bydischarged electrons which flows through a circuit including the firstelectrode, the power source, and the capacitive coupling; and a judgerwhich determines existence of open-circuit and/or short-circuit based onthe current detected by the current detector.
 19. The circuit boardtesting apparatus according to claim 18, further comprising a housingwhich encloses terminals of the plurality of wirings to constitute anairtight closed space, and a depressurizer which depressurizes theclosed space.
 20. The circuit board testing apparatus according to claim18, wherein the circuit board to be tested includes a ground layer, andthe power source is connected to the ground layer so that the groundlayer is capacitively coupled to at least a part of the selected wiringto thereby serve as the second electrode portion.
 21. The circuit boardtesting apparatus according to claim 18, further comprising a connectorwhich connects the power source to a terminal connected with one of theplurality of wirings of the circuit board to be tested so that theterminal serve as the first electrode portion for trapping dischargedelectrons.
 22. The circuit board testing apparatus according to claim18, wherein the circuit board to be tested includes a ground layer, andthe apparatus further comprising a connector which connects the powersource to a terminal connected with one of the plurality of wirings ofthe circuit board to be tested so that the terminal serve as the firstelectrode portion for trapping discharged electrons, and connects thepower source to the ground layer so that the ground layer iscapacitively coupled to at least a part of the selected wiring tothereby serve as the second electrode portion.
 23. A circuit boardtesting apparatus for testing a plurality of wirings formed on a circuitboard, each wiring having a first and second terminals and the firstterminals of the wirings being exposed on one surface of the circuitboard, comprising: an electromagnetic wave irradiator which collectivelyirradiates the first terminals of the wirings with electromagnetic waveto discharge electrons from the first terminals by photoelectric effect;an electrode arranged to trap the discharged electrons; a selector forselecting one of the wirings; a voltage supplier which produces adifference of electric potential between the electrode and the secondterminal of a selected wiring so that the electrode has an electricalpotential higher than that of the second terminal of the selectedwiring; a current detector which detects a current caused by electronstrapped by the electrode portion to flow through the selected wiring viathe electrode; and a judger which judges continuity and/or short-circuitof the selected wiring based on the detected current.
 24. The circuitboard testing apparatus according to claim 23, wherein the voltagesupplier includes a power source having a first pole connected to theelectrode and the second pole connected to the second terminal of thetarget wiring, and the selector includes a switch arrangement forelectrically connects the second terminals of the wirings other than theselected wiring, to the first pole of the power source.
 25. The circuitboard testing apparatus according to claim 23, wherein the voltagesupplier includes a power source having a first pole connected to theelectrode and a second pole connected to the second terminal of thetarget wiring, and the selector has a switch arrangement forelectrically connecting the second terminal of the selected wiring tothe second pole of the power source by way of the current detector, andthe respective second terminals of the wirings other than the selectedwiring to the second pole of the power source bypassing the currentdetector.
 26. The circuit board testing apparatus according to claim 25,wherein the selector includes a switch to select one state where thesecond terminal of the selected wiring is electrically connected to thesecond pole of the power source by way of the current detector and thesecond terminals of the wirings other than the selected wiring areelectrically connected to the second pole of the power source bypassingthe current detector, and another state where the second terminal of theselected wiring is electrically connected to the second pole of thepower source by way of the current detector and the second terminals ofthe wiring other than the selected wiring are electrically connected tothe first pole of the power source.
 27. The circuit board testingapparatus according to claim 23, further comprising a housing whichencloses the second terminals of the plurality of wirings to form anairtight closed space, and a depressurizer which depressurizes theclosed space.
 28. The circuit board testing apparatus according to claim27, wherein the upper wall of the housing is transparent, theelectromagnetic wave irradiator is located above the housing toirradiate the first terminals through the transparent wall, and theelectrode is formed on the housing in the manner allowing the passage ofthe electromagnetic wave through the transparent wall.
 29. A circuitboard testing apparatus for testing a plurality of wirings formed on acircuit board, each wiring having a first and second terminals and thefirst terminals of the wirings being exposed on one surface of thecircuit board, comprising: an electromagnetic wave irradiator whichcollectively irradiates the first terminals of the wirings withelectromagnetic wave to discharge electrons from the first terminals byphotoelectric effect; a power source having a first pole and a secondpole, the potential at the first pole being higher than at the secondpole; a switch arrangement for normally connecting the second terminalsof all the wirings with the first pole of the power source andconnecting the second terminal of one of the wirings to the second poleof the power source while the electromagnetic wave irradiatorirradiating the first terminals, and a current detector which detects acurrent caused by electrons trapped by the first terminals of thewirings other than the selected wiring connected with the second pole ofthe power source, and flows through the selected wiring; and a judgerwhich judges continuity and/or short-circuit of the selected wiringbased on the detected current.
 30. A method for testing continuityand/or short-circuit of wirings formed on a circuit board, each wiringhaving a first and second terminals, the method comprising the steps of:irradiating the fist terminal of a wiring with electromagnetic wave todischarge electrons from the terminal into a space by photoelectriceffect; trapping the discharged electrons by an electrode having apotential higher than that at the second terminal of the wiring to allowa current caused by the trapped electrons to flow from the secondterminal through the wiring; and judging continuity and/or short-circuitof the wiring based on the current flowing through the wiring.
 31. Themethod according to claim 30, further comprising the steps of: enclosingthe space into which electrons are discharged; and depressurizing theclosed space, those steps being carried out before the irradiation step.32. The method according to claim 30, wherein the electromagnetic waveis selectively and successively irradiated onto the first terminals ofthe wirings one by one, and an electrical potential difference isproduced between the electrode and the second terminal of the selectedwiring in such a manner that the electrode has a potential higher thanthat of the selected wiring.
 33. The method according to claim 30,wherein the electromagnetic wave is selectively and successivelyirradiated onto the one terminals of the wirings one by one, and adifference of electric potential is produced between the electrode and asecond terminal of the wiring adjacent to the selected wiring in such amanner that the electrode has a potential higher than that of the secondterminal of the wiring adjacent to the selected wiring.
 34. The methodaccording to claim 30, wherein the electromagnetic wave is selectivelyand successively irradiated onto the first terminals of the wirings, anda difference of electric potential is produced between the electrode andthe second terminal of the selected wiring or between the electrode andthe second terminal of the wiring adjacent to the selected wiring insuch a manner that the electrode has a potential higher than that ofsecond terminal of the selected wiring or the wiring t adjacent to theselected wiring.
 35. A method for testing a plurality of wirings formedon a circuit board, each wiring including a terminal formed on a surfaceof the circuit board and a conductive portion formed on the surface ofthe circuit board or inside the circuit board and electrically connectedto the terminal, the method comprising the steps: irradiating anelectromagnetic wave onto the terminal of a wiring to dischargeelectrons from the terminal into a space by photoelectric effect;trapping the discharged electrons by a first electrode having anelectrical potential higher than that of the wiring; allowing a currentcaused by the trapped electrons to flow through the wiring via acapacitive coupling formed by the wiring and second electrode connectedto the first electrode; and judging continuity and/or short-circuit ofthe wiring based on the current flowing through the wiring.
 36. Themethod according to claim 35, wherein the space into which the electronsare discharged is air-tightly enclosed, and the method furthercomprising the step of depressurizing the closed space.
 37. The methodaccording to claim 35, wherein the circuit board includes a ground layercapacitively coupled with the conductor of the target wiring, and themethod further comprising the step of connecting a power source to theground layer to allow the current caused by the trapped electrons toflow through the conductive portion via the capacitive coupling to theground layer.
 38. A method for testing a plurality of wirings formed ona circuit board, each wiring including a first and second terminalsformed on a surface of the circuit board and an electric conductorformed on the surface of the circuit board or inside the circuit boardand electrically connected to the terminals, the method comprising thesteps: irradiating an electromagnetic wave onto a first terminal of atarget wiring to discharge electrons from the first terminal into aspace by photoelectric effect; trapping the discharged electrons by afirst electrode having an electric potential higher than that of thetarget wiring; allowing a current caused by the trapped electrons andflowing into the conductor via a capacitive coupling of the conductorand a second electrode connected to the first electrode; detecting afirst current flowing through the conductor while the first terminal isbeing irradiated; irradiating an electromagnetic wave onto a secondterminal of the target wiring to discharge electrons from the secondterminal into the space by the photoelectric effect; trapping thedischarged electrons by the first electrode having an electricalpotential higher than that of the target wiring; allowing a currentcaused by the trapped electrons to flow through the conductor viacapacitive coupling of the conductor and a second electrode; detecting asecond current flowing through the conductor while the second terminalis being irradiated; and judging based on the first and second currentsthe continuity between the first and second terminals.
 39. The methodaccording to claim 38, wherein the first current and the second currentare integrated to judge, based on change of their respective integrationfor a time period, the continuity between the first and secondterminals.
 40. A method for testing a circuit board formed with aplurality of wirings, each wiring including a terminal formed on asurface of the circuit board and an electric conductor formed on thesurface of the circuit board or inside the circuit board andelectrically connected to the terminal, the conductor of all the wiringsbeing capacitively coupled with a second electrode, the methodcomprising the steps: irradiating the terminal of the first wiring withelectromagnetic wave to discharge electrons from the terminal into aspace by photoelectric effect; trapping the discharged electrons by afirst electrode having an electric potential higher than that of thetarget wiring; allowing a current caused by the trapped electrons toflow into the conductor of the first wiring via a capacitive coupling ofthe conductors and the second electrode; detecting a first currentflowing through the conductor of the first wiring; irradiating theterminal of a second wiring with electromagnetic wave to dischargeelectrons from the terminal of the second wiring into the space by thephotoelectric effect; trapping the discharged electrons by the firstelectrode having an electrical potential higher than that of the secondwiring; allowing a current caused by the trapped electrons to flowthrough the conductor of the second wiring via capacitive coupling ofthe conductors and the second electrode; detecting a second currentflowing through the conductor of the second wiring; and judgingshort-circuit between the first and second wirings based on the firstand second current.
 41. A method for testing a continuity and/orshort-circuit of a plurality of wirings formed on a circuit board, eachwiring having a first and second terminals, and the first terminals ofthe wirings being exposed on one surface of the circuit board, themethod comprising the steps of: collectively irradiating the firstterminals of the wirings with electromagnetic wave to dischargeelectrons from the first terminals by photoelectric effect; selecting awiring from the wirings; trapping the discharged electron by anelectrode, while applying a voltage between the electrode and theselected wiring such that the voltage at the electrode is higher thanthat of the voltage at the selected wiring; detecting a current causedby electrons trapped by the electrode and flowing through the selectedwiring via the electrode; and judging continuity and/or short-circuit ofthe selected wiring based on the detected current.
 42. The methodaccording to claim 41, further comprising the steps of enclosing thefirst terminals in an airtight closed space and depressurizing thespace.
 43. The method according to claim 41, further comprising the stepof applying a voltage between the second terminal of the selected wiringand the second terminal of at least one of the wirings other than theselected wiring to detect a short-circuit between the selected wiringand the other wiring.
 44. The method according to claim 41, furthercomprising the step of applying a voltage between the second terminal ofthe target wiring and the second terminal of at least one of the wiringsother than the selected wiring to trap, by the first terminal of theother wiring, electrons discharged from the first terminal of theselected wiring.